AVIF support improvements in GD

[desrosj]

This aims to add proper AVIF support to GD for PHP 8.1+ containers.

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[desrosj]

This should not be merged until #185 is merged into trunk and the HEAD branch for this pull request to avoid making changes to legacy containers unnecessarily.

[desrosj]

I'm getting an error when running the media test group locally to test these containers:

1) Test_Image_Resize_GD::test_resize_avif
imageavif(): avif error - Could not encode image: No codec available

Switching to draft until, I can work through this.

[desrosj]

I think this is ready to go. WordPress/wordpress-develop#9919 shows the tests that were previously skipped for GD as running and passing.

[desrosj]

I just gave another look, and I think this is ready to go. Going to merge and confirm in WordPress/wordpress-develop#trunk.