Split video encoders and image encoders, and add support for WebP on libvlc picture
Just like done in !1060 (merged) for the "encoder" capability, split the "image encoder" capability from the "video encoder", to be able to list the image.c-compatible encoders and avoid probing other video encoders.
Also adds WebP on the libvlc API now that it's been added to VLC in !3078 (merged).
Edited by Alexandre Janniaux