macOS: fix initial Metal drawable size in certain multi-display setups

(cherry picked from commit baca26d727)

src/video/cocoa/SDL_cocoametalview.m

@@ -151,6 +151,9 @@ Cocoa_Metal_CreateView(_THIS, SDL_Window * window)
 
     [view addSubview:newview];
 
+    /* Make sure the drawable size is up to date after attaching the view. */
+    [newview updateDrawableSize];
+
     metalview = (SDL_MetalView)CFBridgingRetain(newview);
 
     return metalview;