scene: remove wlr_client_buffer usage

We want to eventually move away from wlr_client_buffer (see e.g. 1). wlr_scene_buffer is a blocker for this since it uses wlr_client_buffer to grab an already-uploaded texture for a buffer.

Instead, add a private scene_buffer_set_buffer_with_damage_and_texture function to set both the buffer and the texture together. Once we have a proper abstraction for wlr_buffer and wlr_texture tied together (see ideas in 2), we can expose it in our public API.