Move GL renderer interface from backends into struct weston_renderer
Prepare the GL renderer for multi-backend support and align with !1112 (merged) by moving the struct gl_renderer_interface
pointer from the backends into struct weston_renderer
.
This way, a primary backend can initialize the renderer, and secondary backends can later find and use it.
This is split out from !578 (merged).
Edited by Philipp Zabel