Do not use actual render object sub classes for render nodes

[creativecreatorormaybenot]

Description

#38 has shown that render nodes should only have access to the correct members (configure, children, render, renderSize, etc.) and not the standard render object members.

Existing implementations

https://github.com/flutter/flutter/blob/4d7525f05c05a6df0b29396bc9eb78c3bf1e9f89/packages/flutter/lib/src/rendering/custom_paint.dart