feat: visual parity with the original Ghostty (pixel-diff pipeline)

Adds scripts/visual-parity.sh (`just visual-parity`): launches the
installed Ghostty 1.3.1 and the Rust build in an isolated headless
nested weston (GL renderer, Mesa forced, weston-screenshooter via
--debug), with identical isolated config, a deterministic test page
(ASCII, ligatures, all SGR styles and underline kinds, 16/256/truecolor,
box/block/braille/powerline/nerd icons, CJK, emoji, wide chars,
cursor) and a tabs+splits scenario driven through Xwayland, then diffs
the captures (terminal area and chrome separately, heatmaps).

Fixes found by comparing against the original:
- A pane stayed blank after a nested split: the GTK render path called
  draw_frame(false) where Ghostty uses drawFrame(true) (a re-realized
  GLArea has a new swap chain that nothing else redraws).
- Sprite glyph antialiasing now ports z2d's default 4x4 multisample
  rasterizer instead of tiny-skia's analytic coverage (pixels differing
  from Ghostty's reference sprite sheets: 5.76% -> 1.20%, none off by
  more than 96 levels); the sheet test tolerance is tightened to 0.1%.

Result vs Ghostty 1.3.1: test page 0.020% of pixels differ (196 px at
curly-underline ends and diagonal box strokes, max delta 86 levels);
tabs+splits and headerbar/tab bar are bit-identical. Details in
docs/rewrite/notes/visual-parity-VP.md.

Test Plan:
- just visual-parity (needs weston, weston-screenshooter, xdotool)
- cargo test -p ghostty-font (sprite sheet comparison)

crates/ghostty-core/src/surface/mod.rs

@@ -2125,7 +2125,10 @@ impl CoreSurface for Surface {
     fn draw_frame(&self, _default_framebuffer: u32) -> Result<(), CoreError> {
         // GTK's GLArea binds its own FBO before `render`; the renderer
         // queries GL_FRAMEBUFFER_BINDING itself.
-        self.renderer.draw_frame(false)?;
+        // Zig: apprt gtk `glRender` calls `renderer.drawFrame(true)`. Sync
+        // must be true: after a GLArea (re)realize (e.g. a split rebuild) the
+        // swap chain is new and nothing else would redraw an unchanged pane.
+        self.renderer.draw_frame(true)?;
         self.debug_screenshot();
         Ok(())
     }