video/hvif: Add HVIF decoder element

This vector format originates from Haiku, where it is used for icons thanks to its very small file size, in comparison to SVG or PNG.

This plugin is based on the hvif crate, which uses cairo to do the rendering, see https://linkmauve.fr/dev/hvif/

I couldn’t figure out how to get the caps from the src pad, so for now the output is hardcoded to 64×64, but it would be obviously better to render at the requested size. Guidance welcome, this is my first Gstreamer element!