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 <[email protected]>

src/ui/window/pdf_dialogs.rs

@@ -2206,6 +2206,9 @@ pub struct ViewBackgroundState {
     pub image: String,
     pub position: BgPosition,
     pub aspect: bool,
+    /// The image's offset (pixels) and scale, kept from the view's image.
+    pub offset: [f64; 2],
+    pub scale: [f64; 2],
     /// The drawing area colour, shown for no background.
     pub canvas: [u8; 3],
 }