feat(renderer): port the OpenGL backend, shaders and size math

Ports the terminal-independent half of ZIG/src/renderer (OpenGL only):
- opengl/*: buffers, frames, pipelines, render passes, samplers,
  targets, textures and the OpenGL GraphicsAPI backend on top of glow;
  the GTK GLArea glue is replaced by a get_proc_address loader closure
- the GLSL shaders, copied verbatim and embedded; uniform/vertex structs
  as bytemuck Pod types with std140 offsets asserted in tests
- shadertoy custom shaders converted with naga (GLSL in -> validate ->
  GLSL 430 out) instead of glslang + spirv-cross
- size (screen/grid/cell/padding math), cursor style logic, backend
  health, options and message types

The crate opts into unsafe_code = "deny"; unsafe glow calls live only in
opengl/gl.rs. A headless EGL integration test builds every pipeline and
the converted shadertoy shaders on Mesa and reads back rendered pixels.

Note: Zig's shadertoy Uniforms order the selection background/foreground
colors opposite to the GLSL block (likely an upstream bug); kept as in
Zig for parity and recorded in the notes.

Deferred to the next renderer pass: generic, cell, image, link, row,
State, Thread, Overlay (need terminal render state).

Test Plan:
- cargo test -p ghostty-renderer (33 tests incl. tests/egl_headless.rs)

crates/ghostty-renderer/shaders/test_shadertoy_invalid.glsl

@@ -0,0 +1,12 @@
+vec2 curve(vec2 uv)
+{
+	uv = (uv - 0.5) * 2.0;
+	uv *= 1.1;
+	uv.x *= 1.0 + pow((abs(uv.y) / 5.0), 2.0);
+	uv.y *= 1.0 + pow((abs(uv.x) / 4.0), 2.0);
+	uv  = (uv / 2.0) + 0.5;
+	uv =  uv *0.92 + 0.04;
+	return uv;
+}
+
+// Missing mainImage!