Support AVIF images

[NexusXe]

AVIF images, though somewhat rare, provide better quality per size than legacy JPEG. Supporting this format for future and current use would be helpful, especially for applications that take images from the web. In many cases, it's faster to get and decode AVIF than it is to do the same for a JPEG, whilst providing rather significant fidelity improvements.

Also, support of AVIF/AV1 opens the door for future hardware acceleration, once such hardware reaches the mainstream (especially in mobile SOCs!)

[Jumprocks1]

osu!framework uses ImageSharp to load textures/images, so once ImageSharp supports AVIF, I'm sure it will also be supported here. The discussion in the ImageSharp repo is here https://github.com/SixLabors/ImageSharp/discussions/1320.