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, 10 insertions, 0 deletions

diff --git a/shaders/post.vert.glsl b/shaders/post.vert.glsl
new file mode 100644
index 00000000..86739a65
--- /dev/null
+++ b/shaders/post.vert.glsl
@@ -0,0 +1,10 @@
+#version 450
+
+// A post pass (docs/render-pipeline.md §6): one fullscreen triangle from
+// gl_VertexIndex, no vertex buffer. The Shadertoy fragment shader works from
+// gl_FragCoord, y down, as its fragCoord.
+
+void main() {
+    vec2 p = vec2(float((gl_VertexIndex << 1) & 2), float(gl_VertexIndex & 2));
+    gl_Position = vec4(p * 2.0 - 1.0, 0.0, 1.0);
+}