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_focus.glsl

@@ -0,0 +1,41 @@
+// Test shader for iTimeFocus and iFocus
+// Shows border when focused, green fade that restarts on each focus gain
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    vec2 uv = fragCoord / iResolution.xy;
+
+    // Sample the terminal content
+    vec4 terminal = texture2D(iChannel0, uv);
+    vec3 color = terminal.rgb;
+
+    if (iFocus > 0) {
+        // FOCUSED: Add border and fading green overlay
+
+        // Calculate time since focus was gained
+        float timeSinceFocus = iTime - iTimeFocus;
+
+        // Green fade: starts at 1.0 (full green), fades to 0.0 over 3 seconds
+        float fadeOut = max(0.0, 1.0 - (timeSinceFocus / 3.0));
+
+        // Add green overlay that fades out
+        color = mix(color, vec3(0.0, 1.0, 0.0), fadeOut * 0.4);
+
+        // Add border (5 pixels)
+        float borderSize = 5.0;
+        vec2 pixelCoord = fragCoord;
+        bool isBorder = pixelCoord.x < borderSize ||
+                       pixelCoord.x > iResolution.x - borderSize ||
+                       pixelCoord.y < borderSize ||
+                       pixelCoord.y > iResolution.y - borderSize;
+
+        if (isBorder) {
+            // Bright cyan border that pulses subtly
+            float pulse = sin(timeSinceFocus * 2.0) * 0.1 + 0.9;
+            color = vec3(0.0, 1.0, 1.0) * pulse;
+        }
+    } else {
+        // UNFOCUSED: Solid red overlay (no border)
+        color = mix(color, vec3(1.0, 0.0, 0.0), 0.3);
+    }
+
+    fragColor = vec4(color, 1.0);
+}