fftools/ffmpeg_mux: stop rescaling timestamps in of_streamcopy()

This function converts packet timestamps from the input stream timebase
to OutputStream.mux_timebase, which may or may not be equal to the
actual output AVStream timebase (and even when it is, this may not
always be the optimal choice due to bitstream filtering).

Just keep the timestamps in input stream timebase, they will be rescaled
as needed before bitstream filtering and/or sending the packet to the
muxer.

Move the av_rescale_delta() call for audio (needed to preserve accuracy
with coarse demuxer timebases) to write_packet.

Drop now-unused OutputStream.mux_timebase.

fftools/ffmpeg.h

@@ -523,8 +523,6 @@ typedef struct OutputStream {
     /* dts of the last packet sent to the muxing queue, in AV_TIME_BASE_Q */
     int64_t last_mux_dts;
 
-    // the timebase of the packets sent to the muxer
-    AVRational mux_timebase;
     AVRational enc_timebase;
 
     Encoder *enc;