Enable webp support in flatpak environment.

While investigating why eog could open webp and loupe could not, I found out that it was only not working in the flatpak environment, while it worked in a system build.

webp support in eog / gdk-pixpuf is provided by webp-pixbuf-loader. I took the relevant module from eog flatpak json and it now works in the loupe dev env as well. I also added webp to the mime type list.

Baby steps in the direction of animated webp support :)