Add background color customization

[Nutzzz]

It looks like you were thinking about implementing this in #20 , so I figured I'd help out.

EDIT: Oops, I actually based it on #20.

[Nutzzz]

Tests were fixed in #21 .

[Nutzzz]

Hm... Somewhere along the line -t (disabling transparency) was broken with GIF (something to do with palette transparency?). It was working with #18 .

[teramako]

Hm... Somewhere along the line -t (disabling transparency) was broken with GIF (something to do with palette transparency?). It was working with #18 .

I think PR #20 commit (cbd1d7b) fixed it. Can you rebase the main branch and give it a try?
Also, the test workflow failure has been fixed (e140b04).

[Nutzzz]

I think PR #20 commit (cbd1d7b) fixed it. Can you rebase the main branch and give it a try?

You're right, it's mostly fixed, but on the main branch, I've still got an issue where it looks like top-left transparency is being applied in addition to palette background color transparency.

EDIT: This PR seems good.

[teramako]

Nice feature.
🚀