Add avif support to ext/gd

This backports avif support from upstream libgd into bundled libgd
and exposes the functionality through new imagecreatefromavif()
and imageavif() function

Add avif support to ext/gd

This backports avif support from upstream libgd into bundled libgd
and exposes the functionality through new imagecreatefromavif()
and imageavif() functions.

Closes GH-7026.

Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>

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