Reset to first frame

[Wavesonics]

It would be super sweet to get a method to reset a GIF while it's animating to the first frame. Essentially starting it over.

[koral--]

OK, good idea.

[koral--]

Done in 189f60c. I'm not sure how should it exactly work, so I've made several assumptions:

• reset() method only rewinds input source and resets internal state. It doesn't restart stopped animation nor clear the canvas immediately. Animation will effectively be restarted just before rendering next frame.
• If source cannot be rewound then nothing happens. This is applicable to files and some InputStreams only. Byte arrays and ByteBuffers are always rewindable.