Make seeking more efficient

[Breakthrough]

Right now seeking is inefficient. Add a more efficient implementation, but also allow the user to specify a fallback to the older (slow) seeking mode if the input is not seekable. Seeking across multiple videos can make things complicated, so not going to address that (as a workaround can just concatenate the inputs ahead of time with ffmpeg/mkvmerge).

Any community examples of videos with broken seeking or wrong reported duration vs actual duration would be very helpful for testing.