output-swapchain: new helper, take 2

The goal is to simplify wlr_output by moving all of its rendering API into separate helpers. Here is a first step to sunset wlr_output_attach_render(). Instead, compositors call wlr_output_swapchain_acquire(), wlr_renderer_begin_with_buffer() and wlr_output_attach_buffer().

Note that compositors can supply a base struct wlr_output_state. This is useful to allocate a buffer suitable for submission with a modeset, for instance.

Closes: #3079 (closed) References: !3197 (closed)