stillFrame option is a mistake - do more intelligent processing

[turt2live]

Instead of assuming that all gifs will be fine, we should figure it out ourselves. Starting at frame 0, find the most contentful frame after some arbitrary amount of frames (100?).

Further, ensure that the thumbnail is correctly taken and not just an artifact of some awkward gif.

[Sorunome]

Also, if the disposal method is not 0 we would actually have to render the gif up to the point of the still frame ourselves