speed up mouse moving

Converted default cursor into a smaller version(24x24). Together with
the change that reduces refreshes, new cursor can work almost smoothly
now.

One thing to remember is that cursor files can only have several fixed
sizes, namely 16x16, 24x24, 32x32, 48x48 etc. Sizes like 21x21 will
break the AND map, which specifies transparent area.

Next thing to do is to rearrange some code.