Fix emission_weight default for lightbulb example

Author: portsmouth

examples/open_pbr_lightbulb.mtlx

@@ -4,6 +4,7 @@
     <input name="surfaceshader" type="surfaceshader" nodename="open_pbr_surface_surfaceshader" />
   </surfacematerial>
   <open_pbr_surface name="open_pbr_surface_surfaceshader" type="surfaceshader">
+    <input name="emission_weight" type="float" value="1.0" />
     <input name="emission_color" type="color3" value="1.000, 0.415, 0.099" />
     <input name="emission_luminance" type="float" value="10000" />
   </open_pbr_surface>