Show loading indicator when video is buffering (#1441)

## Description
Shows a loading indicator and slightly dim the screen when playback is
buffering.

Showing the indicator is delayed in case the buffering doesn't take very
long.

Dev note: the changes to `MpvPlayer` need to be ported to
wholphin-extensions as well.

### Related issues
Closes #1411

### Testing
Emulator

## Screenshots

[buffering.webm](https://github.com/user-attachments/assets/dd848696-d6de-4f68-94e4-901bb5db1185)

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/util/mpv/MpvPlayer.kt

@@ -620,6 +620,16 @@ class MpvPlayer(
                 }
                 notifyListeners(EVENT_IS_PLAYING_CHANGED) { onIsPlayingChanged(!value) }
             }
+
+            MPVProperty.PAUSED_FOR_CACHE -> {
+                Timber.v("paused-for-cache %s", value)
+                playbackState.update {
+                    it.copy(
+                        state = if (value) STATE_BUFFERING else it.state,
+                    )
+                }
+                notifyListeners(EVENT_PLAYBACK_STATE_CHANGED) { onPlaybackStateChanged(playbackState.load().state) }
+            }
         }
     }