This change fixes some cases in which PTS could be less than DTS.

Additionally, a new parameter, b_dts_compress, enables DTS compression.
DTS compression eliminates negative DTS (i.e. initial delay) due to B-frames.
The algorithm changes timebase in order to avoid duplicating DTS.
Currently, in x264cli, only the FLV muxer uses it.  The MP4 muxer doesn't need it, as it uses an EditBox instead.