rtpenc: Merge the h264 and hevc packetizers

They share a great deal of common structure; only a few minor
bits in the headers differ.

This also fixes an off-by-one in sending of the last fragment
of large HEVC nals (where it previously sent len+2 bytes, even
if it should have been len+RTP_HEVC_HEADERS_SIZE aka len+3).

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