MaterialX: Connect geometry_thin_walled flag to surface (#250)

Connect `geometry_thin_walled` flag to `surface`.

Author: jstone-lucasfilm

reference/open_pbr_surface.mtlx

@@ -79,7 +79,7 @@
            doc="The color of the emitted light." />
     <input name="geometry_opacity" type="float" value="1" uimin="0" uimax="1" uiname="Opacity" uifolder="Geometry"
            doc="The opacity of the entire material." />
-    <input name="geometry_thin_walled" type="boolean" value="false" uiname="Thin Walled" uifolder="Geometry" uiadvanced="true"
+    <input name="geometry_thin_walled" type="boolean" value="false" uniform="true" uiname="Thin Walled" uifolder="Geometry" uiadvanced="true"
            doc="If true the surface is double-sided and represents an infinitesimally thin shell. Suitable for extremely geometrically thin objects such as leaves or paper." />
     <input name="geometry_normal" type="vector3" defaultgeomprop="Nworld" uiname="Normal" uifolder="Geometry"
            doc="Input geometric normal" />
@@ -621,6 +621,7 @@
       <input name="bsdf" type="BSDF" nodename="fuzz_layer" />
       <input name="edf" type="EDF" nodename="emission_edf" />
       <input name="opacity" type="float" interfacename="geometry_opacity" />
+      <input name="thin_walled" type="boolean" interface="geometry_thin_walled" />
     </surface>
 
     <!-- Output -->