display: don't apply color-scale-via-lighting to shader inputs

This isn't working properly at the moment, since the state gets set for the *next* object, causing significant artifacts.  I wasn't entirely sure whether to restore the "proper" behavior or not, since applications may be relying on one or the other behavior, but enabling this feature for shaders is a minefield and it's a lot better to just tell people to use p3d_ColorScale instead.

panda/src/display/graphicsStateGuardian.cxx

@@ -1433,12 +1433,7 @@ fetch_specified_part(Shader::ShaderMatInput part, InternalName *name,
       // Apply the default OpenGL lights otherwise.
       // Special exception for light 0, which defaults to white.
       string basename = name->get_basename();
-      if (basename == "color" || basename == "diffuse") {
-        t.set_row(3, _light_color_scale);
-        return &t;
-      } else if (basename == "specular") {
-        return &LMatrix4::ones_mat();
-      }
+      return &LMatrix4::ones_mat();
     }
     return fetch_specified_member(NodePath(), name, t);
   }
@@ -1551,7 +1546,6 @@ fetch_specified_member(const NodePath &np, CPT_InternalName attrib, LMatrix4 &t)
     nassertr(light != nullptr, &LMatrix4::ident_mat());
     if (node->is_ambient_light()) {
       LColor c = light->get_color();
-      c.componentwise_mult(_light_color_scale);
       t.set_row(3, c);
     } else {
       // Non-ambient lights don't currently have an ambient color in Panda3D.
@@ -1570,7 +1564,6 @@ fetch_specified_member(const NodePath &np, CPT_InternalName attrib, LMatrix4 &t)
       t.set_row(3, LColor(0.0f, 0.0f, 0.0f, 1.0f));
     } else {
       LColor c = light->get_color();
-      c.componentwise_mult(_light_color_scale);
       t.set_row(3, c);
     }
     return &t;