View backgrounds: image colours as stored, viewport style name, adjust automation

- The background image texture is not an sRGB view: the image is drawn with
  its own colours, as the solid and gradient colours are.
- An entered paper-space viewport shows its named visual style (Realistic,
  ...) in the render-mode picker, and a mode picked there is recorded as the
  viewport's visual style.
- The view_background automation action takes offset=x,y and scale=x,y.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/window/pdf_dialogs.rs

@@ -122,6 +122,9 @@ pub enum PdfDialogMsg {
     BgBrowse,
     BgPosition(BgPosition),
     BgAspect(bool),
+    /// An image background's offset (pixels) and scale.
+    BgOffset([f64; 2]),
+    BgScale([f64; 2]),
     BgApply,
 }