message: Simplify normal computation in UsdPreviewSurface
  This changelist simplifies the computation of the per-pixel normal in UsdPreviewSurface, leveraging the standard <normalmap> node and removing a dependency on the bitangent stream.
diff: <!-- Node: UsdPreviewSurface -->
  <nodegraph name="IMP_UsdPreviewSurface_surfaceshader" nodedef="ND_UsdPreviewSurface_surfaceshader">
  - <!-- Calculate normal from tangent space input-->
  - <normal name="Nw" type="vector3">
  - <parameter name="space" type="string" value="world"/>
  - </normal>
  - <tangent name="Tw" type="vector3">
  - <parameter name="space" type="string" value="world"/>
  - </tangent>
  - <bitangent name="Bw" type="vector3">
  - <parameter name="space" type="string" value="world"/>
  - </bitangent>
  - <separate3 name="normal_components" type="multioutput">
  - <input name="in" type="vector3" interfacename="normal"/>
  - <output name="outx" type="float" />
  - <output name="outy" type="float" />
  - <output name="outz" type="float" />
  - </separate3>
  - <multiply name="normal_x" type="vector3">
  - <input name="in1" type="vector3" nodename="Tw"/>
  - <input name="in2" type="float" nodename="normal_components" output="outx"/>
  + <!-- Compute the per-pixel surface normal -->
  + <multiply name="scale_normal" type="vector3">
  + <input name="in1" type="vector3" interfacename="normal" />
  + <input name="in2" type="float" value="0.5" />
  </multiply>
  - <multiply name="normal_y" type="vector3">
  - <input name="in1" type="vector3" nodename="Bw"/>
  - <input name="in2" type="float" nodename="normal_components" output="outy"/>
  - </multiply>
  - <multiply name="normal_z" type="vector3">
  - <input name="in1" type="vector3" nodename="Nw"/>
  - <input name="in2" type="float" nodename="normal_components" output="outz"/>
  - </multiply>
  - <add name="normal_xy" type="vector3">
  - <input name="in1" type="vector3" nodename="normal_x"/>
  - <input name="in2" type="vector3" nodename="normal_y"/>
  - </add>
  - <add name="normal_xyz" type="vector3">
  - <input name="in1" type="vector3" nodename="normal_xy"/>
  - <input name="in2" type="vector3" nodename="normal_z"/>
  + <add name="bias_normal" type="vector3">
  + <input name="in1" type="vector3" nodename="scale_normal" />
  + <input name="in2" type="float" value="0.5" />
  </add>
  - <normalize name="surface_normal" type="vector3">
  - <input name="in" type="vector3" nodename="normal_xyz"/>
  - </normalize>
  + <normalmap name="surface_normal" type="vector3">
  + <input name="in" type="vector3" nodename="bias_normal" />
  + </normalmap>
  <!-- Diffuse Layer -->
  <diffuse_brdf name="diffuse_bsdf" type="BSDF">
  