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/lib.rs

@@ -1 +1,22 @@
-//! ghostty-renderer: see docs/rewrite/ARCHITECTURE.md for the module map.
+//! ghostty-renderer: OpenGL renderer building blocks (see
+//! docs/rewrite/ARCHITECTURE.md and docs/rewrite/notes/ghostty-renderer-R1.md).
+//!
+//! Terminal-independent parts of `renderer/`: the OpenGL graphics API backend
+//! and its GPU object wrappers, shaders and their std140 data layouts,
+//! shadertoy custom-shader conversion, size math and cursor style logic.
+
+#![cfg_attr(test, allow(clippy::unwrap_used))]
+
+pub mod backend;
+pub mod cursor;
+pub mod math;
+pub mod message;
+pub mod opengl;
+pub mod options;
+pub mod shadertoy;
+pub mod size;
+
+pub use backend::{Backend, Health};
+pub use cursor::{Style as CursorStyle, style as cursor_style};
+pub use opengl::OpenGL;
+pub use size::{CellSize, Coordinate, GridSize, Padding, ScreenSize, Size};