Auto-generate 3D mesh from convex OBJ if dim=3 in flex.

This requires the user to explicitly specify an origin.

PiperOrigin-RevId: 712483526
Change-Id: If92effcc9bd43ae8b12318044b022965076e02f8

Author: quagla

dm_control/mjcf/schema.xml

@@ -1146,6 +1146,7 @@
             <attribute name="xyaxis" type="array" array_type="float" array_size="6"/>
             <attribute name="zaxis" type="array" array_type="float" array_size="3"/>
             <attribute name="euler" type="array" array_type="float" array_size="3"/>
+            <attribute name="origin" type="array" array_type="float" array_size="3"/>
             <attribute name="dim" type="int" required="true"/>
           </attributes>
           <children>
@@ -1540,6 +1541,7 @@
                 <attribute name="xyaxis" type="array" array_type="float" array_size="6"/>
                 <attribute name="zaxis" type="array" array_type="float" array_size="3"/>
                 <attribute name="euler" type="array" array_type="float" array_size="3"/>
+                <attribute name="origin" type="array" array_type="float" array_size="3"/>
                 <attribute name="dim" type="int" required="true"/>
               </attributes>
               <children>