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/src/backend.rs

@@ -0,0 +1,29 @@
+//! Renderer backend selection (`renderer/backend.zig`).
+
+/// Possible implementations, used for build options.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Backend {
+    Opengl,
+    Metal,
+    Webgl,
+}
+
+impl Backend {
+    /// The default backend for the target. Metal/WebGL are out of scope for
+    /// the Rust port (Linux only), so this is always OpenGL on non-Darwin.
+    pub fn default_for_target() -> Backend {
+        if cfg!(target_vendor = "apple") {
+            Backend::Metal
+        } else {
+            Backend::Opengl
+        }
+    }
+}
+
+/// The health status of a renderer (`renderer.Health`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(C)]
+pub enum Health {
+    Healthy,
+    Unhealthy,
+}