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/message.rs

@@ -0,0 +1,117 @@
+//! Messages that can be sent to a renderer thread (`renderer/message.zig`).
+use std::sync::Arc;
+
+use ghostty_font::SharedGrid;
+
+use crate::size::Size;
+
+/// Key identifying a font grid in the `SharedGridSet`.
+// TODO(integration): replace with ghostty_font's SharedGridSet key once
+// SharedGridSet is ported (it needs ghostty-config).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct SharedGridKey(pub u64);
+
+/// The renderer-thread derived config (`renderer.Thread.DerivedConfig`).
+// TODO(integration): defined with Thread.zig (deferred; needs ghostty-config).
+#[derive(Debug, Clone, Default)]
+pub struct ThreadDerivedConfig {}
+
+/// The renderer implementation derived config (`Renderer.DerivedConfig`).
+// TODO(integration): defined with generic.zig (deferred; needs ghostty-config).
+#[derive(Debug, Clone, Default)]
+pub struct RendererDerivedConfig {}
+
+/// A flattened highlight (`terminal.highlight.Flattened`), used for search matches.
+// TODO(integration): replace with ghostty_terminal::highlight::Flattened.
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct Flattened {}
+
+/// The messages that can be sent to a renderer thread.
+pub enum Message {
+    /// Purposely crash the renderer. This is used for testing and debugging.
+    /// See the "crash" binding action.
+    Crash,
+
+    /// A change in state in the window focus that this renderer is
+    /// rendering within. This is only sent when a change is detected so
+    /// the renderer is expected to handle all of these.
+    Focus(bool),
+
+    /// A change in the view occlusion state. This can be used to determine
+    /// if the window is visible or not. A window can be not visible (occluded)
+    /// and still have focus.
+    Visible(bool),
+
+    /// Reset the cursor blink by immediately showing the cursor then
+    /// restarting the timer.
+    ResetCursorBlink,
+
+    /// Change the font grid. This can happen for any number of reasons
+    /// including a font size change, family change, etc.
+    FontGrid(FontGridChange),
+
+    /// Changes the size. The screen size might change, padding, grid, etc.
+    Resize(Size),
+
+    /// The derived configuration to update the renderer with.
+    ChangeConfig {
+        thread: Box<ThreadDerivedConfig>,
+        r#impl: Box<RendererDerivedConfig>,
+    },
+
+    /// Matches for the current viewport from the search thread. These happen
+    /// async so they may be off for a frame or two from the actually rendered
+    /// viewport. The renderer must handle this gracefully.
+    SearchViewportMatches(Vec<Flattened>),
+
+    /// The selected match from the search thread. May be None to indicate
+    /// no match currently.
+    SearchSelectedMatch(Option<Flattened>),
+
+    /// Activate or deactivate the inspector.
+    Inspector(bool),
+
+    /// The macOS display ID has changed for the window.
+    MacosDisplayId(u32),
+}
+
+/// Payload of [`Message::FontGrid`].
+pub struct FontGridChange {
+    pub grid: Arc<SharedGrid>,
+
+    /// The key for the new grid. If adopting the new grid fails for any
+    /// reason, the old grid should be kept but the new key should be
+    /// dereferenced.
+    pub new_key: SharedGridKey,
+
+    /// After accepting the new grid, the old grid must be dereferenced
+    /// using this key.
+    pub old_key: SharedGridKey,
+    // TODO(integration): upstream also carries `set: *SharedGridSet`.
+}
+
+impl Message {
+    /// Initialize a change_config message.
+    // TODO(integration): `initChangeConfig(alloc, config)` builds the derived
+    // configs from `config.Config`; add once the derived config types exist.
+    pub fn init_change_config(
+        thread: ThreadDerivedConfig,
+        r#impl: RendererDerivedConfig,
+    ) -> Message {
+        Message::ChangeConfig {
+            thread: Box::new(thread),
+            r#impl: Box::new(r#impl),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn change_config_message_roundtrip() {
+        let m = Message::init_change_config(ThreadDerivedConfig {}, RendererDerivedConfig {});
+        assert!(matches!(m, Message::ChangeConfig { .. }));
+    }
+}