render: introduce wlr_render_rect_pass

Split rectangle rendering out of wlr_render_pass and add a dedicated
wlr_render_rect_pass interface.

Remove the add_rect hook from wlr_render_pass_impl and implement
rectangle rendering separately in the pixman, GLES2 and Vulkan
renderers.

include/wlr/render/wlr_renderer.h

@@ -26,6 +26,10 @@ struct wlr_fbox;
  * A renderer for basic 2D operations.
  */
 struct wlr_renderer {
+	struct wlr_render_rect_pass *rect_pass;
+
+	void *data;
+
 	// Capabilities required for the buffer used as a render target (bitmask of
 	// enum wlr_buffer_cap)
 	uint32_t render_buffer_caps;