.saveAlphaFlag ignored when saving WebP or AVIF

[cmb69]

Neither gdImageWebpCtx() nor gdImageAvifCtx() care about .saveAlphaFlag, but always save the full alpha channel, while gdImagePngCtxEx() heeds that flag.

For details see php/php-src#10055. I can port that PR to libgd, but want to be sure that we actually classify this as bug, and if so which branch the fix should target. Given the lack of documentation about the alpha flag, and that WebP and AVIF behave this way since support has been introduced, we probably don't want to fix this in a revision. As such I suggest to fix it in the next minor (if there'll be a 2.4), or major (3.0).

[vapier]

sounds like a bug fix to me