wmv2: move 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. Fate tests for all wmv2-related tests change, because (for some obscure reason) they forced use of the MPEG IDCT. You would get the same changes previously by not using -idct simple in the fate test (or replacing it with -idct auto).
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