cocoa: fix rendering artefacts in retina displays

Set the correct contentsScale in the CALayer instead of using
the default (1.0) which causes rendering artefacts in retina
displays due to using a smaller bitamp than the display size.

See: https://developer.apple.com/documentation/quartzcore/calayer/1410746-contentsscale?language=objc

Fix: #3942
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7828>

subprojects/gst-plugins-base/gst-libs/gst/gl/cocoa/gstglwindow_cocoa.m

@@ -746,6 +746,10 @@ close_window_cb (gpointer data)
     return NO;
 }
 
+- (void)viewDidChangeBackingProperties {
+    self.layer.contentsScale = [[self window] backingScaleFactor];
+}
+
 @end
 
 void