Fix sheen roughness in Standard Surface to glTF PBR translation (AcademySoftwareFoundation#2643)

closes AcademySoftwareFoundation#2642 

**Sheen roughness** in glTF PBR is squared before the `sheen_bsdf` node compare to standard surface implementation.
In this PR the translation graph is updated to take the sqrt of the sheen roughness from standard surface to make sure the intended sheen roughness is translated properly.

Author: bowald

libraries/bxdf/translation/standard_surface_to_gltf_pbr.mtlx

@@ -187,6 +187,9 @@
       <input name="in1" type="float" interfacename="sheen_roughness" />
       <input name="in2" type="float" value="0" />
     </ifgreater>
+    <sqrt name="sqrt_sheen_roughness" type="float">
+      <input name="in" type="float" nodename="sheen_roughness" />
+    </sqrt>
 
     <!-- Clearcoat -->
     <ifequal name="clearcoat" type="float">
@@ -257,7 +260,7 @@
     <output name="thickness_out" type="float" nodename="thickness" />
     <output name="attenuation_color_out" type="color3" nodename="attenuation_color" />
     <output name="sheen_color_out" type="color3" nodename="sheen_color" />
-    <output name="sheen_roughness_out" type="float" nodename="sheen_roughness" />
+    <output name="sheen_roughness_out" type="float" nodename="sqrt_sheen_roughness" />
     <output name="clearcoat_out" type="float" nodename="clearcoat" />
     <output name="clearcoat_roughness_out" type="float" nodename="clearcoat_roughness" />
     <output name="emissive_out" type="color3" nodename="emissive" />