Fixed bug 1148 - SDL window white upon first appearing

To be consistent with other platforms, we'll use black as the background color.

src/video/cocoa/SDL_cocoawindow.m

@@ -690,6 +690,7 @@ Cocoa_CreateWindow(_THIS, SDL_Window * window)
         }
     }
     nswindow = [[SDLWindow alloc] initWithContentRect:rect styleMask:style backing:NSBackingStoreBuffered defer:NO screen:screen];
+    [nswindow setBackgroundColor:NSColor.blackColor];
 
     /* Create a default view for this window */
     rect = [nswindow contentRectForFrameRect:[nswindow frame]];