Add an API to capture/release the mouse cursor

[postcasio]

Currently the lack of capturing can be worked around by constantly resetting the mouse position, but it's an imperfect solution because the mouse can be moved and clicked too fast for the game to realize it has left the window.

This is provided by allegro with these functions: https://liballeg.org/a5docs/trunk/mouse.html#al_grab_mouse

However, they are not currently supported on MacOS.