Cocoa: Fix call to NSWindow from non-main threads

glfwSwapBuffers may be called by any thread but NSWindow may not.

Bug introduced by c3ca88055f and reported
by @crujose.

src/cocoa_window.m

@@ -322,6 +322,14 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
     _glfwInputWindowFocus(window, GLFW_FALSE);
 }
 
+- (void)windowDidChangeOcclusionState:(NSNotification* )notification
+{
+    if ([window->ns.object occlusionState] & NSWindowOcclusionStateVisible)
+        window->ns.occluded = GLFW_FALSE;
+    else
+        window->ns.occluded = GLFW_TRUE;
+}
+
 @end