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.

README.md

@@ -132,6 +132,8 @@ information on what to include when reporting a bug.
  - [Win32] Bugfix: The window hint `GLFW_MAXIMIZED` did not move or resize the
    window (#1499)
  - [Cocoa] Bugfix: `glfwSetWindowSize` used a bottom-left anchor point (#1553)
+ - [Cocoa] Bugfix: Window remained on screen after destruction until event poll
+   (#1412)
  - [X11] Bugfix: The CMake files did not check for the XInput headers (#1480)
  - [X11] Bugfix: Key names were not updated when the keyboard layout changed
    (#1462,#1528)

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
 }