sdl.c: support 32 bpp cursors

sdl.c: support 32 bpp cursors

Hello,
currently when a 32 bpp cursor gets defined the result is all-black in
the areas that are not transparent (you'll get a 32 bpp cursor if you
use my previous patch to allow vmware_vga to use a 32 bpp framebuffer).
This is because the switch in sdl.c lacks a 32 bpp case.
The thing I am unsure about though is which byte is the unused one and
should be skipped, the first or the last - for the black-and-white
cursors I tested it doesn't make a difference...

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Andrzej Zaborowski <balrogg@gmail.com>