Closes GH-14 where flashing text in Mix mode would not flash to the
transparent background.
The bundled QtGifImage does not expose a way to change the DisposalMode in
giflib and thus it is fixed at 0 or DISPOSAL_UNSPECIFIED. This is a quick and
dirty change to fix it to 2 or DISPOSAL_BACKGROUND for our needs.
A proper fix would be to fork QtGifImage and add a more accessible API call
to choose the DisposalMode value.
