WIP: Add AVIF support #258

Closes #210

I didn't test whether the deprecation warning fix breaks TinyIB for PHP 5.5

On second thought, the deprecation warning fix won't break TinyIB for PHP 5.5 since the current code base uses the new variable syntax already, see this code piece for reference.

Thanks for submitting this. I will review and merge ASAP. Frustrating that PHP not only changed the variable parsing syntax but removed the "old" way entirely. Oh well.

I forgot to add fallback images. 😆 I should've read #210 before writing this.

Fallback images exist now. I'll sleep over it, refactor my code and the encompassing scope. And see what I missed.

Should I revert my cosmetic createThumbnail changes? I'm too tired rn to untagle my slip-up here, but maybe tomorrow or so.

Also, is there some sort of code style? I feel like the use of single and double quotes is kinda random. Or maybe I'm too tired to see the big picture, I dunno.

Thanks again for submitting this. I've looked over your changes more closely and there seem to be a lot of changes beyond adding AVIF support. It would be helpful if you could update this PR to only contain the changes necessary to add AVIF support. If you would like to submit additional changes to TinyIB, please submit them as additional separate PRs.

I've looked over your changes more closely and there seem to be a lot of changes beyond adding AVIF support.

What changes are you referring to? Besides the refactored image processing code in inc/functions.php.