qpeg: resolve conflicts (FIXME/TESTME).

 - current is an attempt to honor changes in a0db7424
 - need to be tested with fate + should be tested with V-codecs/QPEG/Clock.avi
 - potential overlapping refdata and dst memcpy in qpeg_decode_inter()
 - on a side note, AVFrame ABI issue + no proper AVFrame init (since no
   alloc with the API) but that is problematic in the fork as well