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 <noreply@anthropic.com>

src/app/update/viewport.rs

@@ -1084,6 +1084,9 @@ impl OpenCADStudio {
         // viewport, the picker drives that viewport entity's own
         // render mode; the model-layout tab style is untouched.
         if self.tabs[i].scene.set_active_viewport_render_mode(mode) {
+            self.tabs[i]
+                .scene
+                .set_active_visual_style_named(crate::modules::view::visual_style::style_name_for(mode));
             self.tabs[i].scene.bump_geometry_no_blocks();
             self.tabs[i].dirty = true;
             self.command_line