Cleanup DrawNode drawing methods

[smoogipoo]

Curious what people's thoughts are on this.

Right now, we have a dichotomy between the implementations of Draw() and DrawOpaqueInternal() where one batches everything including children inside Draw() but the other exposes DrawOpaqueInternalSubtree() for this purpose.

This results in other weirdness, like Draw() being public and DrawOpaqueInternal() being protected. As a consumer of the framework, there is no way to correctly implement DrawOpaqueInternal() if needing to draw a child, prior to this PR.

My eventual goal is to be able to wrap both of these draw functions in some way that indicates to the renderer what DrawNode is currently being drawn, for example something like...

class DrawNode
{
    protected internal static void DrawOther(DrawNode node, IRenderer renderer)
    {
        using (renderer.BeginDrawNode(node))
            node.Draw();
    }
}

This is a breaking change.

vNext

DrawNode.Draw() has been made protected

In order to match the signature of DrawOpaqueInternal(), Draw() has been made protected.

DrawNodes which draw others from within themselves need to make use of helper methods:

- public override void Draw(IRenderer renderer)
+ protected override void Draw(IRenderer renderer)
{
    base.Draw(renderer);

-   other.Draw(renderer);
+   DrawOther(other, renderer);
}

[peppy]

Fine to my eye.