gl-renderer: call supports() automatically in create()

gl-renderer: call supports() automatically in create()

This means compositors don't need to call supports() manually and
create() will just return -1 in the failure case as before. This also
means we can deal with the case of eglGetProcAddress returning
non-NULL but not actually being available at runtime.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>