EGL backend availability requires both EGL extensions and Wayland
interfaces to be present, so we will need to consider multiple backends
during initialization.
As a preliminary work, move the egl_backend to its own struct so that we
can have more than one backend at any given time.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
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