Fix failing GIF rendering on mobile

[Zulko]

Motivation:

It looks like Gix works OK on mobile devices, up to the stage where you render the final GIF, where it just does nothing. This is very probably due to the fact that GIF generation uses Web Workers, and these are disabled in mobile navigators.

Specs

• GIF generation works on mobiles
• Any advantage of web workers, such as nice speed of rendering, are preserved on non-mobile platforms.