Refactor PBR texture layers from separate sub-elements to a single layer sub-element.

Add new element to PyMJCF schema.

PiperOrigin-RevId: 690115286
Change-Id: I016cb7624f93f8a80c7ed6191655b1b8432ef128

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -917,49 +917,10 @@
             <attribute name="vflip" type="keyword" valid_values="false true"/>
           </attributes>
           <children>
-            <element name="rgb" repeated="false">
+            <element name="layer" repeated="true">
               <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="normal" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="occlusion" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="roughness" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="metallic" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="opacity" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="emissive" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="orm" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
-              </attributes>
-            </element>
-            <element name="rgba" repeated="false">
-              <attributes>
-                <attribute name="texture" type="reference" required="false" reference_namespace="texture"/>
+                <attribute name="texture" type="reference"/>
+                <attribute name="role" type="keyword" valid_values="rgb normal occlusion roughness metallic opacity emissive orm rgba"/>
               </attributes>
             </element>
           </children>