Add an option to solidify complex which will make faces which have thickness
controlled by vertex weights flat/even, and parallel to their original face.
For each face it uses the minimal weight assigned to its vertices to control
the thickness.
This will help users for example in architecture or basic CAD design by finally
making solidify work there at all if altering thickness is needed.
Differential Revision: https://developer.blender.org/D7340
Reviewed and minor cleanups by Batien Montagne (@mont29).
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