macosx: use black color for detached window background

This avoids grey or transparent flashes on start

modules/gui/macosx/MainWindow.m

@@ -1860,8 +1860,8 @@ static VLCMainWindow *_o_sharedInstance = nil;
 {
     [self setAcceptsMouseMovedEvents: YES];
 
+    [self setBackgroundColor: [NSColor blackColor]];
     if (b_dark_interface) {
-        [self setBackgroundColor: [NSColor clearColor]];
         [self setOpaque: NO];
         [self display];
         [self setHasShadow:NO];