[AD] Mouse acceleration

["Chris Jones" <allegdev@xxxxxxxxxx>]

Hello,

It seems that the Windows mouse driver does its own mouse acceleration, but
that this is not configurable in any way except through the allegro.cfg
file.

Would there be any objections to updating the mouse driver struct to include
a set_acceleration function, and adding a global set_mouse_acceleration
function, in order that this could be adjusted in code?

Cheers,
Chris