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/glsl/image.v.glsl

@@ -0,0 +1,47 @@
+#include "common.glsl"
+
+layout(binding = 0) uniform sampler2D image;
+
+layout(location = 0) in vec2 grid_pos;
+layout(location = 1) in vec2 cell_offset;
+layout(location = 2) in vec4 source_rect;
+layout(location = 3) in vec2 dest_size;
+
+out vec2 tex_coord;
+
+void main() {
+    int vid = gl_VertexID;
+
+    // We use a triangle strip with 4 vertices to render quads,
+    // so we determine which corner of the cell this vertex is in
+    // based on the vertex ID.
+    //
+    //   0 --> 1
+    //   |   .'|
+    //   |  /  |
+    //   | L   |
+    //   2 --> 3
+    //
+    // 0 = top-left  (0, 0)
+    // 1 = top-right (1, 0)
+    // 2 = bot-left  (0, 1)
+    // 3 = bot-right (1, 1)
+    vec2 corner;
+    corner.x = float(vid == 1 || vid == 3);
+    corner.y = float(vid == 2 || vid == 3);
+
+    // The texture coordinates start at our source x/y
+    // and add the width/height depending on the corner.
+    tex_coord = source_rect.xy;
+    tex_coord += source_rect.zw * corner;
+
+    // Normalize the coordinates.
+    tex_coord /= textureSize(image, 0);
+
+    // The position of our image starts at the top-left of the grid cell and
+    // adds the source rect width/height components.
+    vec2 image_pos = (cell_size * grid_pos) + cell_offset;
+    image_pos += dest_size * corner;
+
+    gl_Position = projection_matrix * vec4(image_pos.xy, 1.0, 1.0);
+}