Sharing frontend with WebP Convert

[nisbet-hubbard]

Maybe this is already the plan. If not, I’d like to make the case for a shared frontend.

Because it’s known that AVIF tends to perform worse in the lossless mode than WebP when the source file is in PNG (AOMediaCodec/av1-avif#111), a good default for WebP Express would be to use the projected AVIF Convert for lossy conversion but reverts to WebP Convert for lossless, but make it possible to swap libraries and compare results.