There is nothing special about the private API which only consists of
getters for renderer specific backing buffer. Lets them to the regular
.h file and treat them as part of the normal API.
https://bugzilla.gnome.org/show_bug.cgi?id=744932
Right now this just has all of the files in one directory. We'll
be introducing more structure to this in the future, and build
a proper backend system.