pixman-renderer: hold a reference for renderbuffers on the output state list

Now that struct weston_renderbuffer is refcounted, hold a reference for
renderbuffers on the pixman_output_state::renderbuffer_list. This allows
backends to destroy the renderer output state and release renderbuffer
references in any order without running into an assert().

To avoid breaking resizing, We also have to drop the renderbuffer list
during pixman_renderer_resize_output(). The backends have to create new
renderbuffers afterwards.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>