compositor: make renderer optional

This is a first step towards moving texture uploading out of
wlr_compositor.

This commit allows compositors to opt-out of the texture uploading
by passing a NULL wlr_renderer. An immediate user of this is
gamescope, which currently implements a stub wlr_renderer just to
make wlr_compositor happy.

include/wlr/types/wlr_compositor.h

@@ -90,7 +90,7 @@ struct wlr_surface_output {
 
 struct wlr_surface {
 	struct wl_resource *resource;
-	struct wlr_renderer *renderer;
+	struct wlr_renderer *renderer; // may be NULL
 	/**
 	 * The surface's buffer, if any. A surface has an attached buffer when it
 	 * commits with a non-null buffer in its pending state. A surface will not
@@ -168,7 +168,7 @@ struct wlr_renderer;
 
 struct wlr_compositor {
 	struct wl_global *global;
-	struct wlr_renderer *renderer;
+	struct wlr_renderer *renderer; // may be NULL
 
 	struct wl_listener display_destroy;
 
@@ -305,6 +305,9 @@ void wlr_surface_unlock_cached(struct wlr_surface *surface, uint32_t seq);
 /**
  * Create the wl_compositor global, which can be used by clients to create
  * surfaces and regions.
+ *
+ * If a renderer is supplied, the compositor will create struct wlr_texture
+ * objects from client buffers on surface commit.
  */
 struct wlr_compositor *wlr_compositor_create(struct wl_display *display,
 	struct wlr_renderer *renderer);

types/wlr_compositor.c

@@ -370,6 +370,10 @@ static void surface_apply_damage(struct wlr_surface *surface) {
 		}
 	}
 
+	if (surface->renderer == NULL) {
+		return;
+	}
+
 	struct wlr_client_buffer *buffer = wlr_client_buffer_create(
 			surface->current.buffer, surface->renderer);
 
@@ -699,8 +703,12 @@ static struct wlr_surface *surface_create(struct wl_client *client,
 	pixman_region32_init(&surface->input_region);
 	wlr_addon_set_init(&surface->addons);
 
-	wl_signal_add(&renderer->events.destroy, &surface->renderer_destroy);
+	if (renderer != NULL) {
-	surface->renderer_destroy.notify = surface_handle_renderer_destroy;
+		wl_signal_add(&renderer->events.destroy, &surface->renderer_destroy);
+		surface->renderer_destroy.notify = surface_handle_renderer_destroy;
+	} else {
+		wl_list_init(&surface->renderer_destroy.link);
+	}
 
 	return surface;
 }