Bevel Extrude Module for OpenSCAD

This is a module that uses the OpenSCAD minkowski() function to allow you to extrude an arbitrary 2D shape with a bevelled edge on top.
If you have concave corners, then you may want to specify a number of facets on the curved bevel that is created using the $fn= argument.
It is a bit slow because OpenSCAD's minkowski seems to take a while.
The arguments are:
* height (How high the 2d shape is extruded).
* bevel_depth (How low the bevel is cut into the top of the extruded object).
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