message: Add guards for negative colors in Standard Surface
  This changelist adds guards for negative color inputs to the Standard Surface graph, addressing edge cases where out-of-gamut color inputs would trigger undefined shader behavior.
diff: <input name="in1" type="float" nodename="coat_gamma_multiply" />
  <input name="in2" type="float" value="1.0" />
  </add>
  - <power name="coat_affected_diffuse_color" type="color3">
  + <max name="base_color_nonnegative" type="color3">
  <input name="in1" type="color3" interfacename="base_color" />
  + <input name="in2" type="float" value="0.0" />
  + </max>
  + <power name="coat_affected_diffuse_color" type="color3">
  + <input name="in1" type="color3" nodename="base_color_nonnegative" />
  <input name="in2" type="float" nodename="coat_gamma" />
  </power>
  - <power name="coat_affected_subsurface_color" type="color3">
  + <max name="subsurface_color_nonnegative" type="color3">
  <input name="in1" type="color3" interfacename="subsurface_color" />
  + <input name="in2" type="float" value="0.0" />
  + </max>
  + <power name="coat_affected_subsurface_color" type="color3">
  + <input name="in1" type="color3" nodename="subsurface_color_nonnegative" />
  <input name="in2" type="float" nodename="coat_gamma" />
  </power>
  