Post chain: Shadertoy passes compiled by glslc; Crt, Ripple and Glitch rewritten as Shadertoy files

Stage 9 of docs/render-pipeline.md. src/gui/Post.zig runs the chain over
the finished frame: each pass reads the one before, ping-pong textures
between, the last writes the window; an empty chain is the direct path.
The prefix (shaders/post/prefix.glsl) is ghostty's uniform block, name
for name and offset for offset (tested against a copy of its Uniforms, the
selection colours by their GLSL names), at SDL GPU's bindings, y down.

A user's file (Shader <path>) is compiled by glslc with compileGlsl's own
flags on a thread of its own, taken in at the next loop step; errors name
the file's own lines; a failed compile keeps the last good pipeline;
glslc missing is said once. The bundled passes compile with the build,
through the same prefix.

Crt, Ripple and Glitch are Shadertoy files, rewritten: no barrel (input is
identity, tested), everything in linear light and dithered, keyed to iTime.
scene_effects, crt.zig and crt.frag.glsl are gone; the core keeps no clock
for the chain. ShaderAnimation off|on|always drives redraw level A: the
chain alone over the retained frame (34 us CPU a redraw, measured).

Shared files touched: pardes.zig (nextWake loses the scene line,
disableSceneEffects empties the chain, two tests), builtins.zig (one switch
arm), ninep/ctl.zig (two switch arms), macos.zig (flags from the chain),
gui.zig. Not touched: Messages.zig, mouse.zig, detached/*, host_io, tty.

1 files changed, 127 insertions, 0 deletions

diff --git a/shaders/prebuilt/post-glitch.frag.glsl b/shaders/prebuilt/post-glitch.frag.glsl
new file mode 100644
index 00000000..46538452
--- /dev/null
+++ b/shaders/prebuilt/post-glitch.frag.glsl
@@ -0,0 +1,127 @@
+#version 450
+// pardes's Shadertoy prefix (docs/render-pipeline.md §6), put before every
+// post pass: a bundled one in shaders/post/ at build time, a user's file at
+// run time. The uniform names, types and std140 order are ghostty 1.3.2's
+// (src/renderer/shaders/shadertoy_prefix.glsl), so a shader written for
+// ghostty runs here; only the bindings are SDL GPU's (a fragment shader's
+// textures in set 2, its uniform blocks in set 3). Y is down everywhere, as
+// on ghostty's Metal renderer: fragCoord, iChannel0's uv and the cursor
+// rectangles share one top-left space, and iCurrentCursor is (left x,
+// BOTTOM-edge y, width, height) in framebuffer pixels.
+
+layout(set = 3, binding = 0, std140) uniform Globals {
+    uniform vec3  iResolution;
+    uniform float iTime;
+    uniform float iTimeDelta;
+    uniform float iFrameRate;
+    uniform int   iFrame;
+    uniform float iChannelTime[4];
+    uniform vec3  iChannelResolution[4];
+    uniform vec4  iMouse;
+    uniform vec4  iDate;
+    uniform float iSampleRate;
+    uniform vec4  iCurrentCursor;
+    uniform vec4  iPreviousCursor;
+    uniform vec4  iCurrentCursorColor;
+    uniform vec4  iPreviousCursorColor;
+    uniform int   iCurrentCursorStyle;
+    uniform int   iPreviousCursorStyle;
+    uniform int   iCursorVisible;
+    uniform float iTimeCursorChange;
+    uniform float iTimeFocus;
+    uniform int iFocus;
+    uniform vec3  iPalette[256];
+    uniform vec3  iBackgroundColor;
+    uniform vec3  iForegroundColor;
+    uniform vec3  iCursorColor;
+    uniform vec3  iCursorText;
+    uniform vec3  iSelectionForegroundColor;
+    uniform vec3  iSelectionBackgroundColor;
+};
+
+// pardes's own: 1 when the window is opaque, so a pass cannot punch holes
+// in it through its alpha.
+layout(set = 3, binding = 1, std140) uniform PardesPost {
+    float pardesOpaque;
+};
+
+#define CURSORSTYLE_BLOCK        0
+#define CURSORSTYLE_BLOCK_HOLLOW 1
+#define CURSORSTYLE_BAR          2
+#define CURSORSTYLE_UNDERLINE    3
+#define CURSORSTYLE_LOCK         4
+
+layout(set = 2, binding = 0) uniform sampler2D iChannel0;
+
+layout(location = 0) out vec4 _fragColor;
+
+#define texture2D texture
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord);
+void main() {
+    mainImage(_fragColor, gl_FragCoord.xy);
+    if (pardesOpaque > 0.5) _fragColor.a = 1.0;
+}
+
+// The shader's own lines count from 1, so glslc's errors name them.
+#line 1
+// Glitch: now and then the signal slips. Every couple of seconds a short
+// burst tears a few horizontal bands sideways and pulls their red and blue
+// apart; the bands are sized in pixels and the burst eases in and out over
+// a fraction of a second, so it reads as one event rather than a flicker,
+// and it looks the same at 60 or 144 Hz because every step is keyed to
+// iTime, not to frames. Between bursts the page is left exactly as it is.
+// A torn band's brightness dips in linear light. It is a Shadertoy file:
+// pardes puts its prefix before it.
+
+const float PERIOD = 2.4;           // seconds between bursts, on average
+const float BURST = 0.28;           // seconds a burst lasts
+const float STEP_HZ = 24.0;         // how often a burst re-tears
+const float SHIFT_PX = 14.0;
+const float SPLIT_PX = 2.5;
+const float DIP = 0.12;
+
+float hash(vec2 p) {
+    vec3 q = fract(vec3(p.xyx) * 0.1031);
+    q += dot(q, q.yzx + 33.33);
+    return fract((q.x + q.y) * q.z);
+}
+
+vec3 toLinear(vec3 c) {
+    return mix(pow((c + 0.055) / 1.055, vec3(2.4)), c / 12.92, lessThanEqual(c, vec3(0.04045)));
+}
+
+vec3 toSrgb(vec3 c) {
+    c = max(c, vec3(0.0));
+    return mix(1.055 * pow(c, vec3(1.0 / 2.4)) - 0.055, c * 12.92, lessThanEqual(c, vec3(0.0031308)));
+}
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    vec2 size = iResolution.xy;
+    vec2 uv = fragCoord / size;
+    // Which burst window this is, when in it the burst starts, and how far
+    // into the burst we are: 0 outside one.
+    float window = floor(iTime / PERIOD);
+    float start = window * PERIOD + hash(vec2(window, 7.0)) * (PERIOD - BURST);
+    float t = (iTime - start) / BURST;
+    float live = step(0.0, t) * step(t, 1.0);
+    float envelope = live * sin(3.14159265 * clamp(t, 0.0, 1.0));
+    if (envelope <= 0.0) {
+        fragColor = texture(iChannel0, uv);
+        return;
+    }
+    // Bands of 6 to 40 px, re-cut at STEP_HZ during the burst; about one
+    // band in four tears.
+    float tick = floor(iTime * STEP_HZ);
+    float band_px = mix(6.0, 40.0, hash(vec2(tick, 3.0)));
+    float band = floor(fragCoord.y / band_px);
+    float roll = hash(vec2(band, tick));
+    float tear = smoothstep(0.72, 0.9, roll) * envelope;
+    float shift = (hash(vec2(tick, band + 11.0)) - 0.5) * 2.0 * SHIFT_PX * tear;
+    vec2 at = vec2(fragCoord.x + shift, fragCoord.y);
+    vec2 split = vec2(SPLIT_PX * tear, 0.0);
+    vec4 center = texture(iChannel0, at / size);
+    vec3 col = vec3(texture(iChannel0, (at + split) / size).r, center.g, texture(iChannel0, (at - split) / size).b);
+    col = toSrgb(toLinear(col) * (1.0 - DIP * tear));
+    fragColor = vec4(col, center.a);
+}