cocoa: set sRGB colorspace on nswindow

This makes the colorspace match across different graphics APIs. By
default, OpenGL was getting a much more saturated colorspace (maybe
Display P3?) and it was looking very different from the rendering done
by Metal or MoltenVK.

src/video/cocoa/SDL_cocoawindow.m

@@ -1743,6 +1743,8 @@ Cocoa_CreateWindow(_THIS, SDL_Window * window)
         return SDL_SetError("%s", [[e reason] UTF8String]);
     }
 
+    [nswindow setColorSpace:[NSColorSpace sRGBColorSpace]];
+
 #if MAC_OS_X_VERSION_MAX_ALLOWED >= 101200 /* Added in the 10.12.0 SDK. */
     /* By default, don't allow users to make our window tabbed in 10.12 or later */
     if ([nswindow respondsToSelector:@selector(setTabbingMode:)]) {