[PATCH] Implement software DXT1 compression

This patch adds conversion functions from WINED3DFMT_B8G8R8_UNORM,
WINED3DFMT_B5G5R5A1_UNORM and WINED3DFMT_B5G5R5X1_UNORM to
WINED3DFMT_DXT1. It uses code slighty modified from libtxc_dxtn. It fixes
at least LEGO Stunt Rally, probably other games too. I didn't implement
dxt3 and dxt5 since I haven't got any apps to test, but if someone has
similar problems (black textures) with them and wants to test, let me
know, and I will add them as well. I would also like some feedback about
the patent situation, e.g. if the patch can be mainlined as it is or if it
would be better to load libtxc_dxtn directly at runtime.
Alberto