perf(scene): extend viewport scissor to hatch / image / wipeout passes (Phase 1.4)

Wire pass already uses `WireModel.vp_scissor` to clip viewport-projected
geometry to its paper-space viewport rect; hatch / image / wipeout
passes drew across the whole canvas. When a future paper-space path
projects fills/images into a viewport, they would bleed past the
viewport frame.

Add `vp_scissor: Option<[f32; 4]>` to HatchModel and ImageModel
(mirroring WireModel), forward it through HatchGpu / ImageGpu, and add
per-pass `compute_*_scissors` / `set_scissor_rect` machinery. Factored
the wire-side scissor-projection math into a shared `project_scissor`
helper. Field defaults to None at every constructor site — no producer
sets it yet, so this is plumbing for the paper-viewport fill projection
that Phase 1.4 unblocks without changing current visual output.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/scene/image_model.rs

@@ -23,6 +23,10 @@ pub struct ImageModel {
     ///   [2] origin + U*W + V*H (top-right)
     ///   [3] origin + V*H (top-left)
     pub corners: [[f32; 3]; 4],
+    /// Optional world-space XY rect [x0, y0, x1, y1] for paper-space
+    /// viewport clipping. Mirrors `WireModel.vp_scissor` /
+    /// `HatchModel.vp_scissor`.
+    pub vp_scissor: Option<[f32; 4]>,
 }
 
 impl ImageModel {
@@ -56,6 +60,7 @@ impl ImageModel {
             height,
             opacity,
             corners,
+            vp_scissor: None,
         })
     }
 }