plug-ins: in file-gegl, use the accurate load/save GEGL operation…
… rather than generic "gegl:load" and "gegl:save". In particular, it means using "gegl:rgbe-load|save" for RGBE images and "gegl:exr-save" for EXR exporting. Without this, we could encounter weird run cases where for instance, we would detect a RGBE image through the file magic number in GIMP, redirect the load to file-gegl, but "gegl:load" only relies on file extension. So if the file extension was not ".hdr", "gegl:load" could redirect to a different handler operation meant for another format, hence break proper loading. If no extension was matched, it could even redirect to a fallback handler, such as Image Magick. This breaks loading or saving images which we would be otherwise able to load/save. And it could also have some security implications. So let's fix this by setting the accurate operations to use for each specific file formats we want to support through the file-gegl plug-in.