MaterialX: metal edge tint should not be multiplied by specular_weight (#240)

Fixes a bug in the MtlX graph. The edge-tint color for the `generalized_schlick_bsdf` should not be multiplied by `specular_weight`, as the whole lobe is multiplied by `specular_weight` as well.

Author: portsmouth

reference/open_pbr_surface.mtlx

@@ -410,22 +410,18 @@
       <input name="in1" type="color3" interfacename="base_color" />
       <input name="in2" type="float" interfacename="base_weight" />
     </multiply>
-    <multiply name="metal_edgecolor" type="color3">
-      <input name="in1" type="color3" interfacename="specular_color" />
-      <input name="in2" type="float" interfacename="specular_weight" />
-    </multiply>
     <generalized_schlick_bsdf name="metal_bsdf" type="BSDF">
       <input name="weight" type="float" interfacename="specular_weight" />
       <input name="color0" type="color3" nodename="metal_reflectivity" />
-      <input name="color82" type="color3" nodename="metal_edgecolor" />
+      <input name="color82" type="color3" interfacename="specular_color" />
       <input name="roughness" type="vector2" nodename="main_roughness" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
       <input name="tangent" type="vector3" interfacename="geometry_tangent" />
     </generalized_schlick_bsdf>
     <generalized_schlick_bsdf name="metal_bsdf_tf" type="BSDF">
       <input name="weight" type="float" interfacename="specular_weight" />
       <input name="color0" type="color3" nodename="metal_reflectivity" />
-      <input name="color82" type="color3" nodename="metal_edgecolor" />
+      <input name="color82" type="color3" interfacename="specular_color" />
       <input name="roughness" type="vector2" nodename="main_roughness" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
       <input name="tangent" type="vector3" interfacename="geometry_tangent" />