Clean up mouse cursor hiding.

In some places, just leave the mouse cursor visible
instead of hiding and showing it in different screens.
Simplify credits timing, and fix an egregious stutter.
Make AGUP draw to gui_screen instead of screen.
Call allegro_exit() at exit.
Remove mouse speed adjustment options.
Fix linking with Allegro in non-default linker path.
Delete unneeded RGB_MAP, COLOR_MAPs.
Factor out al_set_blender calls.
Avoid CPU hogging in most places.