View backgrounds: reference blend, image adjust, layouts, cleanup, style name

- Gradients: the middle colour sits (horizon - 0.5) viewport widths from the
  centre and blends over height viewport heights each side; two colours
  blend over a viewport height each side.
- Image backgrounds: offset in device pixels (right, down), negative scale
  mirrors, fitted with aspect ratio covers the viewport; the dialog keeps an
  image's offset and scale.
- Layouts: a model-space view restored on a sheet asks for the viewport,
  enters it and gives it the view's camera, background and visual style;
  inside a viewport the view goes there; a paper-space view's background is
  kept on the sheet viewport but not drawn; floating viewports draw their
  background.
- Background objects no view or viewport uses any more are erased; a view
  without a background restored where the viewport already is leaves the
  viewport's background.
- The viewport's named visual style (Realistic, ...) shows by name and the
  render-mode picker records its style on the viewport record.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/view/render.rs

@@ -434,8 +434,10 @@ impl shader::Primitive for Primitive {
             // The scene works in logical pixels; an image background keeps
             // its size in device pixels.
             let mut uniforms = vp.uniforms;
-            uniforms.background_image_transform[2] /= scale.max(1e-3);
-            uniforms.background_image_transform[3] /= scale.max(1e-3);
+            // The image's size and offset are in device pixels.
+            for value in &mut uniforms.background_image_transform {
+                *value /= scale.max(1e-3);
+            }
             inner.upload_uniforms(device, queue, &uniforms);
 
             // ── Scene-render cache ────────────────────────────────────────
@@ -2557,20 +2559,18 @@ impl Scene {
         if viewport.paper_sheet {
             let handle = self.current_layout_sheet_viewport_handle();
             if let Some(EntityType::Viewport(value)) = self.document.get_entity(handle) {
-                return build(
-                    value.visual_style_handle,
-                    value.brightness,
-                    value.contrast,
-                    value.background_handle,
-                );
+                // The sheet keeps its page and desk: a paper-space view's
+                // background is kept with the layout but not drawn.
+                return build(value.visual_style_handle, value.brightness, value.contrast, Handle::NULL);
             }
         } else if viewport.handle.is_valid() {
             if let Some(EntityType::Viewport(value)) = self.document.get_entity(viewport.handle) {
+                let flat = value.render_mode == codec::entities::ViewportRenderMode::Wireframe2D;
                 return build(
                     value.visual_style_handle,
                     value.brightness,
                     value.contrast,
-                    value.background_handle,
+                    if flat { Handle::NULL } else { value.background_handle },
                 );
             }
         } else if self.current_layout == "Model" {
@@ -4907,7 +4907,11 @@ impl Scene {
             // (hatched) frame once it settles. Only applied to the on-screen
             // Model / paper content — the paper *sheet* keeps its fills.
             skip_hatch: self.hatch_lod_enabled() && navigating,
-            skip_background: !inst.paper_sheet && self.current_layout != "Model",
+            // A floating viewport shows the page behind it unless it has a
+            // background of its own.
+            skip_background: !inst.paper_sheet
+                && self.current_layout != "Model"
+                && display.background.params[0] < 0.5,
             geometry_epoch: self.geometry_epoch,
             camera_generation: self.camera_generation,
             wire_content_id,