Update SceneObject to allow binding to a named output

[pramsey1234]

scene_rdl2: lib/scene/rdl2/SceneObject.{h,cc} need to allow binding of an attribute to a particular output on a shader object.

Some Implementation Thoughts

• add setBinding version which takes an output name
• getBinding needs to also return the output key or name
  Should there still be a way to get the binding without accepting a name for backward compatibility?
  If so, what should happen if an old getBinding is called on a binding which isn't the only output and isn't the default output (if there is a default)?
• add an an array of output name or key to match mBindings array (one entry per attributes index)

List of Possible Functions

template <typename T> void setBinding(AttributeKey<T> key,
                                      SceneObject* sceneObject, ShaderOutputKey<T> outKey);
void setBinding(const Attribute& attr, SceneObject* sceneObject, ShaderOutput& output);
void setBinding(const std::string& name, SceneObject* sceneObject, const std::string& outName);
template <typename T, typename U> SceneObject* SceneObject::getBinding(AttributeKey<T> key,
                                                                       ShaderOutputKey<U>& outputKey) const;
SceneObject* SceneObject::getBinding(const Attribute& attr, ShaderOutput& output) const;

[pramsey1234]

scene_rdl2: tests/lib/scene/rdl2/TestSceneObject should be updated to test the new functions