G13: Parallax, lapis's dots drift with the text at a quarter of its speed

Parallax on|off (off, a pixel shell's; Chrome.fx bit 3). A body region
carries its first line (Region.line, a file's scroll or a terminal's grid
offset; the wire's region record grows a u32). The GUI offsets each
body's page dots by a quarter of its line in rows, and of G6's glide
offset while one runs, so the pattern drifts with the text and settles
with it; the cell shader reads it from the instance's serial slot, whose
per-cell dissolve noise G7 left unused (the dead noise code goes).

Tests: a scrolled body tells its line; the shift handed to the shader is
a quarter of it within the dots' period. Measured in a capture: four lines
move the dots 13 px (140 px / 4 mod 22). Gates as before, goldens
byte-identical. Feel review: .scratch/render/parallax/.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/surface.zig b/src/surface.zig
index 02e4bb5d..d1ea78b1 100644
--- a/src/surface.zig
+++ b/src/surface.zig
@@ -370,6 +370,10 @@ pub const Region = struct {
     /// focus (Lift, on a spring), a notice's while Lift is on. A pixel shell
     /// casts a soft shadow from it; the tier it is drawn in never changes.
     lift: f32 = 0,
+    /// A body's view: the first line it shows (a file's scroll, a
+    /// terminal's grid offset), for what a pixel shell moves with the text
+    /// (G13's parallax).
+    line: i32 = 0,
     /// Cells the column-move rail takes off the right edge while it runs
     /// there: layers and pixel shells stop short of them, the grid does not.
     clip: u16 = 0,
@@ -451,7 +455,7 @@ pub const Chrome = struct {
     /// A pixel shell's small effects: bit 0 a glow round the body's
     /// selection (G9, SelectionGlow), bit 1 one under the word a look-hover
     /// would open (G11, HoverGlow), bit 2 the bodies' edge shade (G12,
-    /// Occlusion).
+    /// Occlusion), bit 3 the page pattern's parallax (G13, Parallax).
     fx: u8 = 0,
 };