Convert palette images to truecolor before encoding avif with gd

[AlexisSerneels]

Problem

The GD driver fails to save palette (indexed color) images as AVIF:

ErrorException: imageavif(): avif doesn't support palette images
in src/Drivers/Gd/GdDriver.php:172

This shows up in practice with PNG-8 images (common output of PNG optimizers). With spatie/laravel-medialibrary, any AVIF conversion of such an upload fails, so the conversion is never generated.

The WebP branch already handles this by calling imagepalettetotruecolor() before imagewebp(), but the AVIF branch does not.

Fix

Call imagepalettetotruecolor() before imageavif(), in both save() and base64(), mirroring the WebP handling.

Tests

Added two tests using the existing palette-webp.png fixture: saving to AVIF and encoding to AVIF base64. Both are skipped when AVIF is not supported, like the existing AVIF tests.

Ran with PHP 8.4 and GD built with libavif: both GD tests fail without the fix and pass with it.

[freekmurze]

Thanks @AlexisSerneels! Great catch on the PNG-8 to AVIF failure with GD, and nice that the fix came with regression tests for both save() and base64(). I tightened the base64 test to also assert the output is AVIF. This is released in 3.9.7.