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).

libavformat/swf.h

@@ -76,6 +76,7 @@ typedef struct SWFContext {
     int tag;
     AVFifoBuffer *audio_fifo;
     AVCodecContext *audio_enc, *video_enc;
+    AVStream *video_st;
 } SWFContext;
 
 extern const AVCodecTag ff_swf_codec_tags[];