Convert internal representation of vertices to double. Also replace remove duplicate vertices with fast hash implementation.

PiperOrigin-RevId: 734516595
Change-Id: I84baa019db4055504c54868c407d7b7645b72c17

Author: kbayes

dm_control/mjcf/schema.xml

@@ -928,6 +928,7 @@
         <element name="mesh" repeated="true">
           <attributes>
             <attribute name="name" type="identifier"/>
+            <attribute name="content_type" type="string" valid_values="model/obj model/stl model/vnd.mujoco.msh"/>
             <attribute name="class" type="reference" reference_namespace="default"/>
             <attribute name="file" type="file" path_namespace="mesh"/>
             <attribute name="scale" type="array" array_type="float" array_size="3"/>