rtpenc_chain: Don't copy the time base to the source stream by default

Only copy it manually in the muxers where it makes sense (rtspenc,
sapenc). Don't touch the original AVStream in movenchint, where
the original AVStream should be kept untouched.

This fixes the normal tracks in RTP hinted files after
abb810db - the hint tracks were ok while the normal media tracks
were broken, noticed by Michael Niedermayer.

This reverts abb810db but achieves the same effect for the other
muxers.

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