Some .gifs are incorrectly "smearing"

[mcpower]

Input	Output

To reproduce:

1. go get github.com/discord/lilliput
2. cd ~/go/src/github.com/discord/lilliput/examples
3. go build
4. ./examples -input input.gif -output output.gif

Alternatively, upload the input gif to Discord to reproduce.

Using GIMP to inspect the output gif, it seems like Lilliput is incorrectly outputting transparent pixels when it shouldn't be.

(The gif provided here is a minimal reproducible example, which is spliced from a longer gif - that's why there are transparent bars in the gif. I can upload the full gif if needed.)