RootCanvasVertex/RootTriangleIndexElement Deprecation

[psuong]

With issue #26 and supporting shaders and runtime - it might be worth deprecating the RootCanvasVertex and RootTriangleIndexElement because a canvas should effectively only be rebuilt, if you need to disable a portion of the UI and if you add something new entirely.

Instead when the canvas needs to be built, just map reduce the elements and build directly into the mesh on the main thread.

[psuong]

Closing this issue because it's still a requirement, but the structs have been renamed.