Patch by Chris Pearce to tweak indexing in ffmpeg2theora to make it more accurate.
Fixes: 1. Use ogg_int64_t to calculate and store presentation times of frames, rather than transitioning through double just to cast back to ogg_int64_t. Prevents floating point rounding errors from making frames sometimes off by 1ms. 2. Calculate the duration of vorbis packets using the vorbis block/window data. This means that we can accurately know what presentation time we'll get if we decode a vorbis packet. Using this and the knowledge that vorbis packets require their preceding packet to decode, we can now accurately calculate the presentation time we'd get if we began decoding from a vorbis page (rather than just taking the previous page's end-time, which doesn't take into account the fact that we can't decode the first vorbis packet).