explicitly clamp subsurface_color to 0->1 range (#270)

Author: BrianSharpe

reference/open_pbr_surface.mtlx

@@ -135,17 +135,18 @@
     </open_pbr_anisotropy>
 
     <!-- Subsurface (thin-walled) -->
-    <max name="subsurface_color_nonnegative" type="color3">
-      <input name="in1" type="color3" interfacename="subsurface_color" />
-      <input name="in2" type="float" value="0.0" />
-    </max>
+    <clamp name="subsurface_color_clamped" type="color3">
+      <input name="in" type="color3" interfacename="subsurface_color" />
+      <input name="low" type="color3" value="0.0, 0.0, 0.0" />
+      <input name="high" type="color3" value="1.0, 1.0, 1.0" />
+    </clamp>
     <oren_nayar_diffuse_bsdf name="subsurface_thin_walled_reflection_bsdf" type="BSDF">
-      <input name="color" type="color3" nodename="subsurface_color_nonnegative" />
+      <input name="color" type="color3" nodename="subsurface_color_clamped" />
       <input name="roughness" type="float" interfacename="base_diffuse_roughness" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
     </oren_nayar_diffuse_bsdf>
     <translucent_bsdf name="subsurface_thin_walled_transmission_bsdf" type="BSDF">
-      <input name="color" type="color3" nodename="subsurface_color_nonnegative" />
+      <input name="color" type="color3" nodename="subsurface_color_clamped" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
     </translucent_bsdf>
     <subtract name="one_minus_subsurface_scatter_anisotropy" type="float">
@@ -168,7 +169,7 @@
       <input name="in2" type="float" interfacename="subsurface_radius" />
     </multiply>
     <subsurface_bsdf name="subsurface_bsdf" type="BSDF">
-      <input name="color" type="color3" nodename="subsurface_color_nonnegative" />
+      <input name="color" type="color3" nodename="subsurface_color_clamped" />
       <input name="radius" type="color3" nodename="subsurface_radius_scaled" />
       <input name="anisotropy" type="float" interfacename="subsurface_scatter_anisotropy" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />