compositor: migrate to stable viewporter.xml

Migrate from wl_scaler to wp_viewporter extension. The viewporter.xml
file is provided by wayland-protocols.

This stops Weston from advertising wl_scaler, and advertises
wp_viewporter instead.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
[Pekka: fix wayland-protocols requirement]
diff --git a/src/compositor.h b/src/compositor.h
index f2a0e4c..9e5155c 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -993,7 +993,8 @@
 
 	/* wl_surface.set_buffer_transform */
 	/* wl_surface.set_scaling_factor */
-	/* wl_viewport.set */
+	/* wp_viewport.set_source */
+	/* wp_viewport.set_destination */
 	struct weston_buffer_viewport buffer_viewport;
 };
 
@@ -1042,7 +1043,7 @@
 	int32_t height_from_buffer;
 	bool keep_buffer; /* for backends to prevent early release */
 
-	/* wl_viewport resource for this surface */
+	/* wp_viewport resource for this surface */
 	struct wl_resource *viewport_resource;
 
 	/* All the pending state, that wl_surface.commit will apply. */