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/Cargo.toml

@@ -6,11 +6,21 @@ license.workspace = true
 
 [dependencies]
 bytemuck = { version = "1.25.2", features = ["derive"] }
+crossbeam-channel = "0.5.17"
+ghostty-config.workspace = true
+ghostty-datastruct.workspace = true
 ghostty-font.workspace = true
+ghostty-input.workspace = true
+ghostty-terminal.workspace = true
+ghostty-unicode.workspace = true
+ghostty-vt.workspace = true
 glow = "0.18.0"
+jpeg-decoder = { version = "0.3.2", default-features = false }
 khronos-egl = { version = "6.0.0", features = ["dynamic"] }
 log = "0.4.34"
 naga = { version = "30.0.1", features = ["glsl-in", "glsl-out", "wgsl-in"] }
+onig = "6.5.3"
+png = "0.18.1"
 thiserror = "2.0.21"
 
 [lints.rust]