Cocoa: Process events after window destruction

On macOS a destroyed window remained on screen until the next time
events were processed.  This makes the behavior more consistent with
other platforms.

Fixes #1412.

src/cocoa_window.m

@@ -962,6 +962,9 @@ void _glfwPlatformDestroyWindow(_GLFWwindow* window)
     [window->ns.object close];
     window->ns.object = nil;
 
+    // HACK: Allow Cocoa to catch up before returning
+    _glfwPlatformPollEvents();
+
     } // autoreleasepool
 }