rtpenc: base max_frames_per_packet on avg_frame_rate, not codec timebase

Fall back to 1 (which is what is used for most cases anyway) when the
framerate is not set.
lavf: add the notimestamps flag to the muxers missing it
riffenc: take an AVStream instead of an AVCodecContext

It will be useful in the following commits.

Also, rename the AVCodecContext pointer name from 'stream' to 'codec'.
lavf: switch to AVStream.time_base as the hint for the muxer timebase

Previously, AVStream.codec.time_base was used for that purpose, which
was quite confusing for the callers. This change also opens the path for
removing AVStream.codec.

The change in the lavf-mkv test is due to the native timebase (1/1000)
being used instead of the default one (1/90000), so the packets are now
sent to the crc muxer in the same order in which they are demuxed
(previously some of them got reordered because of inexact timestamp
conversion).