Make AVIF and WEBP optional, even if CGO is enabled

[farhaven]

Hi!

We're building our binaries with CGO enabled because another dependency forces us to. Since this enables AVIF and WEBP, our code now depends on libwebp and libaom-dev, which in turn means we need to provide them in our build infrastructure.

Since we don't intend to use either AVIF or WEBP in the foreseeable future, would it be possible to build those formats only if, say, build tags avif and webp are set?

I can provide a patch if this is something you'd consider. If not, I think the new dependencies should be mentioned in the repo README file.

[tdewolff]

I've created a new build tag 'formats' to enable those formats!