(graphics): fix proxy geometry generation
The following discussion from !179 (merged) should be addressed:
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@kgaudet started a discussion: I just realized that this won't work with '2D grids' (XY1 grids), e.g. with a full 2x2x1 grid ...
Oh well, don't worry about it, I'll try to resolve this when I'll have some time.
the heuristic to compute the maximum number of vertices needed by the proxy geometry is wrong.