mxfdec: update current edit unit when seeking in mxf files without index tables

Fixes audio packet pts values in some files generated by AVID TRMG 3.01.

Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavf: improve support for AVC-Intra files.

Generate extradata with SPS/PPS based on container dimensions.

Authors of this commit are: Reimar and Thomas Mundt

Signed-off-by: Anton Khirnov <anton@khirnov.net>
h264: free the tables and uninitialize the context on flush

Prevents referencing empty frames when the first packet after the flush
does not contain a frame.