This function changes the cursor image Allegro uses if
select_mouse_cursor() is called but no native operating system cursor
can be used, e.g. because you did not call enable_hardware_cursor().
The cursor argument can be one of:
but not MOUSE_CURSOR_NONE.
The bmp argument can either point to a valid bitmap or it can be NULL.
Passing a bitmap makes Allegro use that image in place of its own
default substitution (should the operating systems native cursor
be unavailable). The bitmap must remain available for the duration
in which it could be used. Passing NULL lets Allegro revert to its
The effect of this function will not be apparent until show_mouse() is