2pass encoding support

[Selur]

Would be nice to have 2pass encoding support to have the ability to aim for a specific file size.

[Ben Waggoner]

It was be particularly nice to be able to reuse the .stats and .mbtree files created at one bitrate for another bitrate or frame size, as x264 allows.

[Steve Borho]

two-pass encoding is now functional on the current tip; the last relevant commit was <<cset b85dbec30cc5>>

[Selur]

Nice! will do some testing in the next days and report if I run into problems

[Selur]

Just realized, there is no command line option to use 2pass encoding,... -> would be nice if one could be added :)