rtpenc: Allow packetizing H263 according to the old RFC 2190

According to newer RFCs, this packetization scheme should only
be used for interfacing with legacy systems.

Implementing this packetization mode properly requires parsing
the full H263 bitstream to find macroblock boundaries (and knowing
their macroblock and gob numbers and motion vector predictors).

This implementation tries to look for GOB headers (which
can be inserted by using -ps <small number>), but if the GOBs
aren't small enough to fit into the MTU, the packetizer blindly
splits packets at any offset and claims it to be a GOB boundary
(by using Mode A from the RFC). While not correct, this seems
to work with some receivers.

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