Gif export seems to only export colors in the current palette

Issue: When exporting as a gif, it seems that colors which aren't part of the palette end up not being in the final gif. Those pixels end up transparent or simply the wrong color.

How to reproduce: Open a png file with `rx file.png` where file.png is an image containing colors that aren't in the starting palette. Split into multiple frames, then export as a gif. Switch to a palette with the required colors, then export as a second gif. The first gif has missing colors, but the second gif is correct.