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/app/commands/layerprops.rs

@@ -891,7 +891,8 @@ impl OpenCADStudio {
                             self.command_line
                                 .push_error(crate::t!("Usage: VIEW DELETE <name>").as_ref());
                         } else {
-                            if self.tabs[i].scene.document.views.remove(&name).is_some() {
+                            if let Some(view) = self.tabs[i].scene.document.views.remove(&name) {
+                                crate::scene::erase_background(&mut self.tabs[i].scene.document, view.background_handle);
                                 self.command_line
                                     .push_output(crate::tf!("View '{}' deleted.", name).as_ref());
                             } else {