Instead of having a pending_fb field on the struct wlr_drm_plane,
move it to struct wlr_drm_connector_state. That way, there's no
risk having a stale pending FB around: the state doesn't survive
across tests and commits.
The cursor is a special case because it's disconnected from the
atomic state: the wlr_backend_impl.set_cursor hook sets the cursor
for the next commit. Move the field to
wlr_drm_connector.cursor_pending_fb.
037b21647b