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 <[email protected]>

src/shaders/background.wgsl

@@ -81,8 +81,12 @@ fn equirect_uv(ndc: vec2<f32>, rotation: f32) -> vec2<f32> {
 fn image_uv(source_uv: vec2<f32>) -> vec2<f32> {
     var uv = source_uv;
     if u.background_image_params.x < 0.5 {
-        uv = (source_uv - u.background_image_transform.xy)
-            / max(abs(u.background_image_transform.zw), vec2<f32>(1e-6));
+        // Not fitted: the image at its own pixel size (times its scale),
+        // centred on the viewport and moved by its offset.
+        let pixel = (source_uv - vec2<f32>(0.5)) * max(u.viewport_size, vec2<f32>(1.0));
+        uv = pixel / max(abs(u.background_image_transform.zw), vec2<f32>(1e-6))
+            + vec2<f32>(0.5) - u.background_image_transform.xy;
+        return select(uv, fract(uv), u.background_image_params.z > 0.5);
     }
     if u.background_image_params.y > 0.5 {
         let image_aspect = max(u.background_image_params.w, 1e-6);