Time changes

I think I finally get how Matroska handles time and why the timestamps
are quantized. See the comments for details.

What is not clear is that it appears that lowering the timestamp
affects the compression quality. It shouldn't. More exploring to do.
Time issues sorted out

Or so I think. Will keep this branch separate for a while until I'm
sure.

One weird thing is that if I lower the TimeCodeScale from 1000000 to
1000 I get some problems with the file written. Maybe a muxer bug?
Added Match Source checkbox

Since matching the dimensions and frame rate of the source is what I
want about 99% of the time.
Bad assert

whoops!
Ogg constant bitrate

Finally figured out the Ogg issue. All it took was for me to write a
whole other plug-in.
Merge remote-tracking branch 'origin/time_changes'