Use wmv2 IDCT for wmv2 fate tests.
Move wmv idct to its own DSP context.

This allows us to remove FF_IDCT_WMV2, which serves no practical purpose
other than to be able to select the WMV2 IDCT for MPEG (or vice versa)
and get corrupt output.
Put FF_IDCT_{VP3,H264,CAVS,BINK,EA,WMV2} under FF_API_IDCT.

This allows us to get rid of them on the next major bump. All of the
above are functionally irrelevant, and most of them are unused, except
the vp3 one, which is used wrongly in the bfin arch optimizations.