[SDL] directfb cursor patch

[Sam Lantinga]

> here is a quick and dirty patch that allows directfb to change the 
> visibility of the cursor. this display backend still has no way to change 
> the actual cursor shape, but being able to set the cursor visibility is a 
> simple change that should offer some benefits.

I've applied your patch to CVS.

Thanks!
	-Sam Lantinga, Software Engineer, Blizzard Entertainment