Fix building with mock-backend and no-subsurfaces

src/backend/mock.rs

@@ -60,6 +60,12 @@ fn create_solid_capture_image(r: u8, g: u8, b: u8) -> CaptureImage {
             .into(),
         ))
         .0,
+        #[cfg(feature = "no-subsurfaces")]
+        image: cosmic::widget::image::Handle::from_pixels(
+            512,
+            512,
+            [r, g, b, 255].repeat(512 * 512),
+        ),
     }
 }