fix(viewport): clip non-rectangular paper viewports to their boundary shape

Paper content viewports with a circle / spline / ellipse clip boundary
drew their model content to the full bounding rectangle instead of the
shape. Each paper content viewport is its own render into its own screen
rectangle, so clipping is done per viewport: the boundary polygon is
projected into that viewport's render-target NDC (paper shape mapped
through the same visible-sub-rect crop as the content, so it stays fixed
to the viewport frame) and stamped into the stencil buffer with an
even-odd Invert, which marks the interior of any shape, convex or not,
without triangulation. Every content pass then draws only where the
stencil is set, so hatch, wire, mesh, 3DFACE, text, image, wipeout and
selection are all clipped uniformly.

Because Invert turns the cleared 0x00 into 0xFF, clipped content draws
with stencil reference 0xFF (unclipped content keeps reference 0). The
shared depth buffer moves to Depth24PlusStencil8 across every pipeline.

Rectangular viewports keep clipping via their own render rectangle and
XCLIP keeps its existing CPU wire clip; neither needs the stencil path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/viewcube.rs

@@ -1187,7 +1187,7 @@ impl ViewCubePipeline {
                 ..Default::default()
             },
             depth_stencil: Some(wgpu::DepthStencilState {
-                format: wgpu::TextureFormat::Depth32Float,
+                format: wgpu::TextureFormat::Depth24PlusStencil8,
                 depth_write_enabled: true,
                 depth_compare: wgpu::CompareFunction::Less,
                 stencil: wgpu::StencilState::default(),
@@ -1319,7 +1319,7 @@ fn create_depth_texture(device: &wgpu::Device, size: Size<u32>) -> wgpu::Texture
         mip_level_count: 1,
         sample_count: 1,
         dimension: wgpu::TextureDimension::D2,
-        format: wgpu::TextureFormat::Depth32Float,
+        format: wgpu::TextureFormat::Depth24PlusStencil8,
         usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
         view_formats: &[],
     })