don't wait in background for UI updates to finish

VLC for iOS WatchKit Extension/VLCNowPlayingInterfaceController.m

@@ -115,7 +115,7 @@
 {
     UIImage *image = [VLCThumbnailsCache thumbnailForManagedObject:file toFitRect:CGRectMake(0., 0., _screenBounds.size.width * _screenScale, _screenBounds.size.height * _screenScale) shouldReplaceCache:NO];
 
-    [self.playElementsGroup performSelectorOnMainThread:@selector(setBackgroundImage:) withObject:image waitUntilDone:YES];
+    [self.playElementsGroup performSelectorOnMainThread:@selector(setBackgroundImage:) withObject:image waitUntilDone:NO];
 }
 
 - (IBAction)playPausePressed {

VLC for iOS WatchKit Extension/VLCPlaylistInterfaceController.m

@@ -218,7 +218,7 @@ typedef enum {
 
     UIGraphicsEndImageContext();
 
-    [array.firstObject performSelectorOnMainThread:@selector(setBackgroundImage:) withObject:newImage waitUntilDone:YES];
+    [array.firstObject performSelectorOnMainThread:@selector(setBackgroundImage:) withObject:newImage waitUntilDone:NO];
 }
 
 //TODO: this code could use refactoring to be more readable