Fix CGL context not being released until later

Fixes #721.

README.md

@@ -113,6 +113,8 @@ does not find Doxygen, the documentation will not be generated.
  - [Cocoa] Bugfix: Connecting a joystick that reports no name would segfault
  - [Cocoa] Bugfix: Modifier flags cache was not updated when window became key
  - [Cocoa] Bugfix: Dead key character composition did not work
+ - [Cocoa] Bugfix: The CGL context was not released until the autorelease pool
+                   was drained by another function
  - [X11] Bugfix: Monitor connection and disconnection events were not reported
  - [X11] Bugfix: Decoding of UTF-8 text from XIM could continue past the end
  - [X11] Bugfix: An XKB structure was leaked during `glfwInit`

src/cocoa_window.m

@@ -1038,6 +1038,9 @@ void _glfwPlatformDestroyWindow(_GLFWwindow* window)
 
     [window->ns.object close];
     window->ns.object = nil;
+
+    [_glfw.ns.autoreleasePool drain];
+    _glfw.ns.autoreleasePool = [[NSAutoreleasePool alloc] init];
 }
 
 void _glfwPlatformSetWindowTitle(_GLFWwindow* window, const char *title)