Elide updates to backing stores when the contents of that layer don't change.

[chinmaygarde]

During custom composition (custom embedders & iOS only today), the backing stores of all layers and rendered into presented on each updates. This is in spite of the fact that some layers may not have need updated during that frame. These updates need to be elided. Once this optimization is in place, FlutterLayer::did_update can sometimes return false. Currently, it is always true.