Starting new clips always causes a delay

[itsmattkc]

While playback performance is generally acceptable now, starting a new clip causes a "seek" which generally causes a noticeably pause in playback. Olive could theoretically detect upcoming clip changes and seek them before they need to be displayed. The built-in frame caching would handle the rest, so that starting the new clip would be essentially seamless. This kind of optimization should come at the same time as addressing #21 (which will likely require detecting multiple clips of the same source and duplicating the format context).

Olive should also dealloc frame caches on clips after they are finished to save memory on particularly large sequences.

[itsmattkc]

A format/codec handling rewrite is coming that should address this, as well as #21

[itsmattkc]

I'm going to merge this issue with #21 because they are fundamentally caused by exactly the same problem (especially after the file handling rewrite in 157a658)