allow clip cursor to game view area for Linux

[cloudziq]

small feature request as in issue title, hoping someone will jump in and make it possible :)

Lack of this feature for Linux completly (or almost) break my game, if I going to bundle it for that platform.
Not to mention others can use it for own advantages
Thanks in advance, cheers

[subsoap]

https://tronche.com/gui/x/xlib/input/XGrabPointer.html

confine_to | Specifies the window to confine the pointer in or None.