extract_shadows responsive coords fix (#17236)

# Objective

`extract_shadows` uses the render world entity corresponding to the
extracted camera when it queries the main world for the camera to get
the viewport size for the responsive viewport coords resolution and
fails. This means that viewport coords get resolved based on a viewport
size of zero.

## Solution

Use the main world camera entity.

crates/bevy_ui/src/render/box_shadow.rs

@@ -259,7 +259,7 @@ pub fn extract_shadows(
             continue;
         };
 
-        let Ok(camera_entity) = mapping.get(camera_entity) else {
+        let Ok(render_entity) = mapping.get(camera_entity) else {
             continue;
         };
 
@@ -326,7 +326,7 @@ pub fn extract_shadows(
                     color: drop_shadow.color.into(),
                     bounds: shadow_size + 6. * blur_radius,
                     clip: clip.map(|clip| clip.clip),
-                    camera_entity,
+                    camera_entity: render_entity,
                     radius,
                     blur_radius,
                     size: shadow_size,