rtph263pdepay: init debug category
rtph263ppay: remove annex-? fields in caps if not present.
rtph263depay: don't drop mode b packets with picture start code

Some buggy payloaders, e.g. rtph263pay, may use mode B for packets
that starts with a picture (or GOB) start code although it's not
allowed. Let's be nice and not drop these packets/frames.
rtph263pay: change log level on bitstream parsing messages
rtph263*pay: Allow all payload numbers.
rtph263: Make sure that RFC2190 header is present.