Sparse intra only decode

[Tjoppen]

This fixes #986. It also brings the handling of rational numbers more in line with how ffmpeg does things, avoiding the use of floating point timestamp calculations.

With this change, if the output fps is lower than the source fps and the source is intra-only, then only packets that are actually necessary to decode are sent to the decoder. Frame threading is still done as before. This is most dramatic when decoding expensive formats like JPEG2000. If the output fps is the same or higher than the source fps then the behavior is as before.