Support seeking in raw EVC streams

[kpchoi]

Seeking in raw EVC streams did not work and could crash the decoder.

• evc_parser: a CRA picture (non-IDR I slice) is a random access point;
  mark it as a key frame so open-GOP streams get more than one index entry
• evcdec: cache SPS/PPS and re-inject them after a seek (raw streams carry
  them only once), flush the BSF, restart the access-unit counter at the
  seek point, and stamp pts shifted by the reorder delay so the core does
  not discard the dts of reordered streams
• libxevd: add a flush callback that recreates the decoder on seek, and
  return EAGAIN instead of an empty frame when an access unit produced no
  frame

Requires mpeg5/xevd#98.

[mpeg5]

LGTM.
This PR is for random access of RAW EVC stream.
The random access for isobmff file is also required later.