implement opacity inheritance via DL Dispatcher delegate

[flar]

This change will shift the implementation of opacity inheritance (used to implement the opacity peephole optimization) from the SKCanvas Dispatcher class to a Dispatcher delegate so that it can be used for either SkCanvas rendering or for rendering via a DisplayList Dispatcher interface.

An optional opacity argument is added to the DisplayList::Dispatch method to conveniently wrap the dispatcher with the delegation class or use a saveLayer if the display list being dispatched cannot inherit opacity.