OSX set cursor position not working in fullscreen mode

The cursor position was not set because the fullscreen window was not considered to be in focus.

src/cocoa_window.m

@@ -60,6 +60,8 @@ static void enterFullscreenMode(_GLFWwindow* window)
 
     [window->ns.view enterFullScreenMode:window->monitor->ns.screen
                              withOptions:nil];
+    
+    _glfwInputWindowFocus(window, GL_TRUE);
 }
 
 // Leave fullscreen mode