Add Script sub-message with uri, material_name

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Author: scpeters

proto/ignition/msgs/scene.proto

@@ -34,6 +34,12 @@ import "ignition/msgs/header.proto";
 
 message Scene
 {
+  message Script
+  {
+    repeated string uri = 1;
+    string material_name = 2;
+  }
+
   /// \brief Optional header data
   Header header       = 1;
 
@@ -53,5 +59,5 @@ message Scene
   bool origin_visual  = 12;
 
   /// \brief Name of shadow caster material.
-  string shadow_caster_material_name = 13;
+  Script shadow_caster_script = 13;
 }