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 <noreply@anthropic.com>

src/scene/pipeline/image_gpu.rs

@@ -53,6 +53,8 @@ struct ImageParams {
 pub struct ImageGpu {
     pub vertex_buffer: wgpu::Buffer,
     pub bind_group: wgpu::BindGroup,
+    /// Mirrors `ImageModel.vp_scissor` — see `HatchGpu.vp_scissor`.
+    pub vp_scissor: Option<[f32; 4]>,
     _texture: wgpu::Texture,
     _sampler: wgpu::Sampler,
     _params_buf: wgpu::Buffer,
@@ -183,6 +185,7 @@ impl ImageGpu {
         Some(Self {
             vertex_buffer,
             bind_group,
+            vp_scissor: model.vp_scissor,
             _texture: texture,
             _sampler,
             _params_buf,