fix(image): stop darkening rasters; X only for broken references

Image textures were created Rgba8UnormSrgb: sampling decoded sRGB to
linear, but the surface isn't sRGB, so every raster (images, OLE, PDF
underlays) rendered visibly dark. Rgba8Unorm passes the texels through
in the same space as every other colour in the app.

The frame wire also always carried the X diagonals — that's the
broken-reference placeholder, and it scribbled over resolvable images.
The diagonals now draw only when the reference doesn't resolve.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scene/pipeline/image_gpu.rs

@@ -94,7 +94,7 @@ impl ImageGpu {
             mip_level_count: 1,
             sample_count: 1,
             dimension: wgpu::TextureDimension::D2,
-            format: wgpu::TextureFormat::Rgba8UnormSrgb,
+            format: wgpu::TextureFormat::Rgba8Unorm,
             usage: wgpu::TextureUsages::TEXTURE_BINDING | wgpu::TextureUsages::COPY_DST,
             view_formats: &[],
         });