buffer: make enum wlr_buffer_cap public
Custom backends and renderers need to implement wlr_backend_impl.get_buffer_caps
and wlr_renderer_impl.get_render_buffer_caps
. They can't if enum wlr_buffer_cap
isn't made public.
This was an oversight I made during the 0.14.0 release. I'd say it would make sense to backport to 0.14.1.