vout_ios2: take into account scale of attached screen

As said in Apple documentation drawRect should not be implemented for view based on opengl es layer.
Instead contentScaleFactor should be changed manually if needed. Underlying opengl es layer will adjust its scale accordingly.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

modules/video_output/ios2.m

@@ -393,10 +393,10 @@ static void OpenglESSwap(vlc_gl_t *gl)
     [super dealloc];
 }
 
-/* we don't get the correct scale factor if we don't overwrite this method */
-- (void)drawRect:(CGRect) rect
+- (void)didMoveToWindow
 {
-    [super drawRect:rect];
+    self.contentScaleFactor = self.window.screen.scale;
+    _bufferNeedReset = YES;
 }
 
 - (void)createBuffers