Animation gif export requires a background colour for transparency #227

There is no need for the transparency colour to be picked. We can fork gifts and

• add a unused colour to the panel (might be a problem with a max palette size? Would also have to make sure the findClosest never returns it. We could potentially store this separately, and only add to the pallet for writing)
• use that colour index as the transparent colour
• when analysing pixels, for every empty empty pixel of the original image set the colour index to the transparent one.