Move this code from headless-backend to a helper library, so it can be
shared with wayland-backend.
gl-renderer.h was missing #pragma once, which made the build fail.
Unfortunately gl-borders needs gl-renderer.h which will attempt to
include EGL headers if gl-renderer is enabled in the build, so we must
get the EGL build flags too, just for the headers.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
