rtpdec: Allow setting the need_parsing field in RTPDynamicProtocolHandler

This allows getting rid of quite a bit of boilerplate in depacketizers.

The default value (initializing need_parsing to 0, aka
AVSTREAM_PARSE_NONE) is the same as it is initialized to by default
in AVStream.

Signed-off-by: Martin Storsjö <martin@martin.st>
