Joystick routines
Unlike keyboard or mouse input, which are usually read through hardware
interrupts by Allegro, joystick input functions have to be polled because
there are no hardware interrupts for them on most platforms. This doesn't
mean that you have to poll the joysticks on each line of code you want to
read their values, but you should make sure to poll them at least once per
frame in your game loop. Otherwise you face the possibility of reading stale
incorrect data.
