Compositors now have more control over how the backend creates its
renderer. Currently all backends create an EGL/GLES2 renderer, so
the necessary attributes for creating the context are passed to a
user-provided callback function. It is responsible for initializing
provided wlr_egl and to return a renderer. On fail, return 0.
Fixes#987
Add a remote display name argument to wlr_wl_backend_create.
If NULL is passed to the wayland backend at all times, creating a
wayland backend *after* the compositor was started up, would require
changing the WAYLAND_DISPLAY environment variable.
