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_crt.glsl

@@ -0,0 +1,59 @@
+// This shader is NOT BUILD INTO Ghostty. This is only here for unit tests.
+
+// Loosely based on postprocessing shader by inigo quilez, License Creative
+// Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
+
+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;
+}
+void mainImage( out vec4 fragColor, in vec2 fragCoord )
+{
+    vec2 q = fragCoord.xy / iResolution.xy;
+    vec2 uv = q;
+    uv = curve( uv );
+    vec3 oricol = texture( iChannel0, vec2(q.x,q.y) ).xyz;
+    vec3 col;
+	float x =  sin(0.3*iTime+uv.y*21.0)*sin(0.7*iTime+uv.y*29.0)*sin(0.3+0.33*iTime+uv.y*31.0)*0.0017;
+
+    col.r = texture(iChannel0,vec2(x+uv.x+0.001,uv.y+0.001)).x+0.05;
+    col.g = texture(iChannel0,vec2(x+uv.x+0.000,uv.y-0.002)).y+0.05;
+    col.b = texture(iChannel0,vec2(x+uv.x-0.002,uv.y+0.000)).z+0.05;
+    col.r += 0.08*texture(iChannel0,0.75*vec2(x+0.025, -0.027)+vec2(uv.x+0.001,uv.y+0.001)).x;
+    col.g += 0.05*texture(iChannel0,0.75*vec2(x+-0.022, -0.02)+vec2(uv.x+0.000,uv.y-0.002)).y;
+    col.b += 0.08*texture(iChannel0,0.75*vec2(x+-0.02, -0.018)+vec2(uv.x-0.002,uv.y+0.000)).z;
+
+    col = clamp(col*0.6+0.4*col*col*1.0,0.0,1.0);
+
+    float vig = (0.0 + 1.0*16.0*uv.x*uv.y*(1.0-uv.x)*(1.0-uv.y));
+	col *= vec3(pow(vig,0.3));
+
+    col *= vec3(0.95,1.05,0.95);
+	col *= 2.8;
+
+	float scans = clamp( 0.35+0.35*sin(3.5*iTime+uv.y*iResolution.y*1.5), 0.0, 1.0);
+
+	float s = pow(scans,1.7);
+	col = col*vec3( 0.4+0.7*s) ;
+
+    col *= 1.0+0.01*sin(110.0*iTime);
+	if (uv.x < 0.0 || uv.x > 1.0)
+		col *= 0.0;
+	if (uv.y < 0.0 || uv.y > 1.0)
+		col *= 0.0;
+
+	col*=1.0-0.65*vec3(clamp((mod(fragCoord.x, 2.0)-1.0)*2.0,0.0,1.0));
+
+    float comp = smoothstep( 0.1, 0.9, sin(iTime) );
+
+	// Remove the next line to stop cross-fade between original and postprocess
+//	col = mix( col, oricol, comp );
+
+    fragColor = vec4(col,1.0);
+}