Enable Zeltner sheen (#217)

This changelist enables Zeltner sheen in the reference implementation of OpenPBR, leveraging the new functionality in MaterialX 1.39.

Additionally, the open_pbr_velvet.mtlx example has been updated to account for the visual differences between Conty-Kulla and Zeltner sheen.

Author: jstone-lucasfilm

examples/open_pbr_velvet.mtlx

@@ -8,6 +8,6 @@
     <input name="specular_roughness" type="float" value="0.8" />
     <input name="fuzz_weight" type="float" value="1" />
     <input name="fuzz_color" type="color3" value="0.4, 0.4, 0.4" />
-    <input name="fuzz_roughness" type="float" value="0.2" />
+    <input name="fuzz_roughness" type="float" value="0.5" />
   </open_pbr_surface>
 </materialx>

reference/open_pbr_surface.mtlx

@@ -558,12 +558,12 @@
     </layer>
 
     <!-- Fuzz Layer -->
-    <!-- TODO: Add a new BSDF node for the selected fuzz model in OpenPBR -->
     <sheen_bsdf name="fuzz_bsdf" type="BSDF">
       <input name="weight" type="float" interfacename="fuzz_weight" />
       <input name="color" type="color3" interfacename="fuzz_color" />
       <input name="roughness" type="float" interfacename="fuzz_roughness" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
+      <input name="mode" type="string" value="zeltner" />
     </sheen_bsdf>
     <layer name="fuzz_layer" type="BSDF">
       <input name="top" type="BSDF" nodename="fuzz_bsdf" />