Improved NV12/NV21 support.

- Fixed PlanarToNV12Wrapper() and made it handle NV21.
- Added yuv2nv12XinC() to handle software scaling.
- Added NV12/NV21 handling to various places.
- Removed NV12 from vf_hue and vf_spp as they don't look like they can
  actually handle it.

Originally committed as revision 14716 to svn://svn.mplayerhq.hu/mplayer/trunk/postproc