Option to override palette for all frames

I've added an option to override palette for all frames. By passing the option `globalPalette` you can provide an `[r,g,b,...]` array, or `true` to use the palette of the first frame.

If it's set to `true`, only one worker is started at first. After the first frame, it's palette is stored, and the remaining workers are spawned.

This PR is based on my previous one with dithering.

You can see it in action here: http://depthy.me/