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/opengl/mod.rs

@@ -51,23 +51,7 @@ pub const MIN_VERSION_MAJOR: u32 = 4;
 pub const MIN_VERSION_MINOR: u32 = 3;
 
 /// `config.AlphaBlending`.
-// TODO(integration): replace with ghostty_config's AlphaBlending.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
-pub enum AlphaBlending {
-    #[default]
-    Native,
-    Linear,
-    LinearCorrected,
-}
-
-impl AlphaBlending {
-    pub fn is_linear(self) -> bool {
-        match self {
-            AlphaBlending::Native => false,
-            AlphaBlending::Linear | AlphaBlending::LinearCorrected => true,
-        }
-    }
-}
+pub use ghostty_config::types_b::AlphaBlending;
 
 #[derive(Debug, thiserror::Error)]
 pub enum Error {
@@ -296,6 +280,12 @@ impl OpenGL {
         Ok(())
     }
 
+    /// The most recently presented target, if any (also set when the blit to
+    /// the default framebuffer is a no-op, e.g. on a surfaceless context).
+    pub fn last_target(&self) -> Option<&Arc<Target>> {
+        self.last_target.as_ref()
+    }
+
     /// Drop the remembered last-presented target (call when the target it
     /// refers to is being replaced, e.g. on resize).
     pub fn clear_last_target(&mut self) {