[x3d-public] TwoSidedMaterial

[Andreas Plesch]

There is movement around TwoSidedMaterial. See

https://github.com/castle-engine/view3dscene/issues/23

and

http://web3d.org/pipermail/x3d-public_web3d.org/2019-December/011594.html

I would vote to keep a version of it since the alternative of manually
doubling the geometry with inverted normals is too awkward for normal users.

There is a need, however, to clear up some confusion. As far as I can judge
there is no need for separateBackColor. It should always be true since this
case appears to be the main point of the node.

separateBackColor=false should be covered by solid=false and regular
material.

solid=false already needs to use inverted normals for rendering back faces.
An inverted normal is actually what makes the other face is a back face.

Also, there needs to be clarification how solid=true and TwoSidedMaterial
interact. I think x3dom lets TwoSidedMaterial win over solid=true in the
intent to render back faces. Most other browser probably let solid=true
win, eg. will not render back faces at all even with a TwoSidedMaterial.

Of note is a time period where TwoSidedMaterial was broken in x3dom. It did
cause quite a few inquiries. The typical workaround was to suggest using
actually two sides each with its own Material, similar to the conversion
path Don had suggested. People not surprisingly preferred to have the node
fixed.
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