When all of the input meshes have no loose edges, the output won't have
any either, but finding that out for the output is much more expensive.
Copy this tag to the output mesh to save processing time when drawing.
In my test creating 1 million instances of a 16 face mesh, this changed
the time for the draw cache loose geometry extraction from 220 ms to
131 ms.
For now, only do this when the loose edges of the inputs are already
cached (primitive nodes, for example). This avoids paying the cost of
calculation when we don't know it will be necessary.
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