Use a different method for gathering frames

[Fauntleroy]

GIFit currently gathers frames by advancing one frame at a time, copying the contents of the <video>, and moving to the next frame. This doesn't work all the time because we can't have nice things in this world. Some videos are encoded... strangely and Chrome's seeking behavior will sometimes get stuck on keyframes... meaning you might get the same frame for 20 frames if you try to seek.

GIFit originally used a "live playback" method, which I'll have to revert to in the near future.

[Fauntleroy]

https://github.com/Fauntleroy/GIFit/issues/37