It was bothering me that the bitmap struct never got zeroed.
al_set_mouse_xy on Windows resulted in the mouse getting set
to the wrong position in windowed modes (for me 3 pixels left of
where it should be on x, didn't check y.) This indicated there's
probably a problem with al_win_get_window_position. This fixes
the al_set_mouse_xy problem but there seems to be another issue.