Up until now, the name of every AVBSFContext for logging purposes was
"AVBSFContext", so that the default logging callback produced output
like "[AVBSFContext @ 0x55813bae92c0] Extradata". This has been changed
to "[trace_headers @ 0x60a000000700] Extradata" by adding an item_name-
function to the AVClass for bitstream filters.

Furthermore, the correct category has been set so that the introductory
part before the actual message (everything before "Extradata" in the
above examples) are displayed in a different colour than the rest.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>