Initial add of some (disabled) SiS 6326 drawing code integrated from Alan Cox's

Initial add of some (disabled) SiS 6326 drawing code integrated from Alan Cox's
last drop I saw, which was in turn based on a code drop of mine.  Texturing,
culling, and several extensions are unimplemented, and some features could
probably be improved. It's untested, but there don't appear to be regressions on
the 300-series code, so I'd like to get these bits in now so that it can be
worked on as not a huge diff.