Fixed Mouse Cursor Messages for Win32

The user cursor is only supposed to be set when the mouse is in the client
area of the window, not its border. I resolved it by intercepting the hit
test window message and checking if the mouse is in the client area and
then setting a boolean value which I use in the set cursor message to
determine whether to set the user cursor or delegate to the default window
process. This is exactly how Game Maker handles it. Tested and confirmed working, please merge when ready.