linux-explicit-synchronization: Allow fences with opaque EGL buffers

Add opaque EGL buffers to the supported buffer types for use with the
explicit synchronization protocol. Opaque EGL buffers rely on the same
EGL implementation in both the compositor and clients, which makes it
straightforward to manage client expectations about fence support for
such buffers.

Also make it clearer that implementations are free to support other
buffer types beyond the required ones.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>