compositor: add a way to disable picking of client surfaces

Surfaces like drag'n'drop icons shouldn't receive events as a normal
surface but are still created by the client so add a way for the
compositor to enable or disable the picking of a client surface.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
diff --git a/src/compositor.c b/src/compositor.c
index 237509f..31e26d8 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -544,7 +544,7 @@
 	struct weston_surface *surface;
 
 	wl_list_for_each(surface, &compositor->surface_list, link) {
-		if (surface->surface.resource.client == NULL)
+		if (!surface->pickable)
 			continue;
 		weston_surface_from_global(surface, x, y, sx, sy);
 		if (0 <= *sx && *sx < surface->geometry.width &&
@@ -1270,6 +1270,8 @@
 		(void (**)(void)) &surface_interface;
 	surface->surface.resource.data = surface;
 
+	surface->pickable = 1;
+
 	wl_client_add_resource(client, &surface->surface.resource);
 }