Draw a centred image background at its pixel size

An image background not fitted to the screen keeps its own pixel size
(times its scale), centred on the viewport and moved by its offset, as
the reference draws it; tiling repeats it from there.

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

src/scene/view/render.rs

@@ -3009,11 +3009,13 @@ impl Scene {
                         background.use_tiling as u8 as f32,
                         image.width as f32 / image.height.max(1) as f32,
                     ];
+                    // Offset, then the image's size in pixels at its scale:
+                    // an image not fitted to the screen keeps its pixel size.
                     result.image_transform = [
                         background.offset.x as f32,
                         background.offset.y as f32,
-                        background.scale.x as f32,
-                        background.scale.y as f32,
+                        background.scale.x as f32 * image.width as f32,
+                        background.scale.y as f32 * image.height as f32,
                     ];
                     result.params[0] = 4.0;
                     result.image = Some(image);