Fix #1227: webp images load as black even with every prerequisite being green during...

Fix #1227: webp images load as black even with every prerequisite being green during compile

https://github.com/BestImageViewer/geeqie/issues/1227
https://github.com/BestImageViewer/geeqie/issues/1076
https://github.com/BestImageViewer/geeqie/pull/1225

Revert to using image-load-webp.cc
This Geeqie webp decoder will take precedence over an installed
webp-pixbuf-loader.

This decoder does not support webp animations.

If the user wants to use an installed webp-pixbuf-loader (e.g. to get
animations), Geeqie must be compiled with the option -Dwebp=disabled.