JBR-6433 Rounded corners of popups disappear after a while

Removed opacity from CAMetalLayer for layers with rounded corners

(cherry picked from commit e9bf36f41a)

src/java.desktop/macosx/native/libawt_lwawt/awt/AWTWindow.m

@@ -2918,6 +2918,7 @@ JNIEXPORT void JNICALL Java_sun_lwawt_macosx_CPlatformWindow_nativeSetRoundedCor
         w.contentView.wantsLayer = YES;
         w.contentView.layer.cornerRadius = radius;
         w.contentView.layer.masksToBounds = YES;
+        w.contentView.layer.opaque = NO;
 
         if (borderWidth > 0) {
             CGFloat alpha = (((borderRgb >> 24) & 0xff) / 255.0);