Move primary wlr_swapchain handling out of backends

[emersion]

Move the allocator/renderer-related code out of backends into the common backend/output code for everything related to the primary buffer. This process shouldn't break any compositor.

The DRM backend has not been migrated yet, because it requires more work.

• Allow backends to be created without get_renderer
• Allow outputs to be created without {attach,rollback}_render
• Create allocator and renderer in wlr_backend
• Create swapchain in wlr_output
• Add support for the Pixman renderer
• Migrate backends
  • Headless
  • Wayland
  • X11
  • DRM (done separately: backend/drm: move primary wlr_swapchain handling out of backend #2903)

References: #1352
Depends on: #2496
Depends on: #2498
Depends on: #2495
Depends on: #2511
Depends on: #2507
Depends on: #2561

[emersion]

• We need to select a CRTC for the connector prior to wlr_output_test/wlr_output_commit, so that we can return a meaningful list of formats out of wlr_output_impl.get_dmabuf_primary_formats.
• Ideally we should completely wire up test-only commits to wlr_output_test to implement the modifier-less fallback path. Although I suppose we can also let the atomic commit fail, just like we currently do internally in the DRM backend. The downside is that a commit failure resets the pending state.

[emersion]

This has been rebased and updated to work with the Pixman renderer.

This is ready for review.