MacOS: Fixed an issue with transparent windows on Mojave

Author: Tom Poole

modules/juce_gui_basics/native/juce_mac_NSViewComponentPeer.mm

@@ -116,7 +116,14 @@ static NSRect flippedScreenRect (NSRect r) noexcept
            #else
             [window setDelegate: window];
            #endif
+
             [window setOpaque: component.isOpaque()];
+
+           #if defined (MAC_OS_X_VERSION_10_14)
+            if (! [window isOpaque])
+                [window setBackgroundColor: [NSColor clearColor]];
+           #endif
+
             [window setHasShadow: ((windowStyleFlags & windowHasDropShadow) != 0)];
 
             if (component.isAlwaysOnTop())