Two improvements for image converter formats:

1. Add AVIF to image file types supported by 'image-convert'.
   The duplicate line for '.webp' is removed since WEBP
   is supported natively now.  But still WEBP is added to
   'imagemagick-enabled-types' for the case when WEBP libs
   are not available.

2. Improve support for XCF by merging all layers.