WIP: buffer: drop wlr_client_buffer

wlr_client_buffer is weird because it's a wlr_buffer implementation used for all wl_buffer types, created on wl_surface.commit and destroyed on the next one. It wraps a wlr_texture and forces some assumptions on the renderer.

Split out of https://github.com/swaywm/wlroots/pull/2851 because that's a pretty intrusive change.