Add weston_surface_has_content()

Just a trivial wrapper to tell you whether or not the surface has valid
content.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1928,6 +1928,9 @@ weston_surface_attach_solid(struct weston_surface *surface,
 void
 weston_buffer_destroy_solid(struct weston_buffer_reference *buffer_ref);
 
+bool
+weston_surface_has_content(struct weston_surface *surface);
+
 void
 weston_view_destroy(struct weston_view *view);
 

libweston/compositor.c

@@ -3609,6 +3609,16 @@ weston_compositor_schedule_repaint(struct weston_compositor *compositor)
 		weston_output_schedule_repaint(output);
 }
 
+/**
+ * Returns true if a surface has a buffer attached to it and thus valid
+ * content available.
+ */
+WL_EXPORT bool
+weston_surface_has_content(struct weston_surface *surface)
+{
+	return !!surface->buffer_ref.buffer;
+}
+
 static void
 surface_destroy(struct wl_client *client, struct wl_resource *resource)
 {