Move layer element to the right place (from texture to material).

PiperOrigin-RevId: 698497943
Change-Id: Ic693eec8aa9542a6eb628e17c40781b01b1a658a

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -915,14 +915,6 @@
             <attribute name="hflip" type="keyword" valid_values="false true"/>
             <attribute name="vflip" type="keyword" valid_values="false true"/>
           </attributes>
-          <children>
-            <element name="layer" repeated="true">
-              <attributes>
-                <attribute name="texture" type="reference"/>
-                <attribute name="role" type="keyword" valid_values="rgb normal occlusion roughness metallic opacity emissive orm rgba"/>
-              </attributes>
-            </element>
-          </children>
         </element>
         <element name="hfield" repeated="true">
           <attributes>
@@ -1003,6 +995,14 @@
             <attribute name="reflectance" type="float"/>
             <attribute name="rgba" type="array" array_type="float" array_size="4"/>
           </attributes>
+          <children>
+            <element name="layer" repeated="true">
+              <attributes>
+                <attribute name="texture" type="reference" required="true"/>
+                <attribute name="role" type="keyword" valid_values="rgb normal occlusion roughness metallic opacity emissive orm rgba" required="true"/>
+              </attributes>
+            </element>
+          </children>
         </element>
       </children>
     </element>