Fix callout box border on Mac

Author: oepavel

JuceLibraryCode/modules/juce_gui_basics/native/juce_mac_NSViewComponentPeer.mm

@@ -138,6 +138,11 @@ static NSRect flippedScreenRect (NSRect r) noexcept
             [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())