Exporting selection not working for GIFs

[dacap]

Exporting the selection for .gif files is not working.

Reported here: https://community.aseprite.org/t/exporting-selection-not-working-for-gifs/17791

Fix this bug: https://igara-studio.sentry.io/share/issue/28ee7c0a1c7d438c8524996d45bf7a00/

[dacap]

I'm kicking this to v1.3-rc4 because it requires several changes which I'd like to test more intensively (adding more automated tests).