feat(renderer): port the generic renderer, renderer thread and state

Ports renderer/{generic,cell,row,link,State,Overlay,image,Thread}.zig:
- generic.rs builds GPU cell buffers from terminal RenderState each
  frame: backgrounds, shaped text runs with glyph atlases and nerd font
  constraints, all cursor styles, decorations via sprites, selection and
  search colors, min-contrast, bold/faint/blink handling, preedit,
  padding extension, background image, custom shader uniforms, link
  hover underlines, scrollbar and health reporting.
- The renderer thread updates CPU-side state only and asks the host to
  redraw; the app runtime calls draw_frame from the GLArea render signal
  (the GL context lives on the GTK thread, as in Ghostty).
- ghostty_renderer::state::{State, SharedState} is the canonical shared
  renderer state (termio's interim copy will switch to it).

Deviations: overlay uses a direct RGBA rasterizer instead of z2d; bg
images decode with png/jpeg-decoder instead of wuffs; zero-length link
matches advance by one char (Zig would loop forever).

Kitty image rendering is implemented in image::State but not yet called
from generic::update_kitty (raced with the terminal-side wiring).

Test Plan:
- cargo test -p ghostty-renderer (69 unit + 9 headless EGL tests that
  render a Terminal through the full pipeline and check pixels)

crates/ghostty-renderer/src/options.rs

@@ -3,14 +3,17 @@ use std::sync::Arc;
 
 use ghostty_font::SharedGrid;
 
+use crate::generic::Host;
 use crate::size::Size;
 
 /// Options for constructing a renderer.
 ///
 /// `config` is the derived renderer configuration (`Renderer.DerivedConfig`).
-// TODO(integration): upstream also carries `surface_mailbox`, `rt_surface`
-// and `thread` (apprt / Thread.zig types). They are added when the generic
-// renderer and the apprt exist; the GL loader closure replaces `rt_surface`.
+///
+/// Zig also carries `rt_surface` (used by the graphics API to prepare the
+/// GL context) and `thread`; in Rust the apprt prepares the GL context
+/// itself (`OpenGL::prepare_context`) before constructing the renderer, and
+/// the thread is wired up by [`crate::thread::Thread`].
 pub struct Options<C> {
     /// The derived configuration for this renderer implementation.
     pub config: C,
@@ -20,4 +23,7 @@ pub struct Options<C> {
 
     /// The size of everything.
     pub size: Size,
+
+    /// The surface mailbox / apprt callbacks (Zig: `surface_mailbox`).
+    pub host: Arc<dyn Host>,
 }