Add GLFW_CENTER_CURSOR

Adds a hint for controlling whether the cursor is centered over newly
created full screen windows.

Fixes #749.
Closes #842.

src/cocoa_window.m

@@ -1096,7 +1096,8 @@ int _glfwPlatformCreateWindow(_GLFWwindow* window,
         if (!acquireMonitor(window))
             return GLFW_FALSE;
 
-        centerCursor(window);
+    if (wndconfig->centerCursor)
+            centerCursor(window);
     }
 
     return GLFW_TRUE;