On servers with the composite extension enabled, visuals with 32-bit depth
have an alpha mask.  This is pretty neat, but SDL needs a bit more work to
handle these properly, so for now, we'll just use 24 bit depth visuals.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401648