Fixes window_center
GM centers using the unscaled region dimensions to center the window so
that window_default or window_center will work in fullscreen mode making
the window centered if the user returns to windowed mode. Tested and
confirmed on GM8.1
Implements keyboard_key
Also fixes keyboard_lastkey so that they both behave the same as GM8.1,
also tested multiple keyboard key combinations and releases to ensure the
behavior is implemented properly. Also fixed for XLIB.
Merge pull request #747 from RobertBColton/master

Keyboard Fixes