Polyhedron_demo: alpha problems on macOS


## Issue Details
Apparently, on macOS, when drawing an item with an alpha value <1, the edges and the faces are not aligned, so there is an item with only the edges, and at some other place the rendering of the "alpha item".
## Environment

* Operating system (Windows/Mac/Linux, 32/64 bits): macOS
* CGAL version: 5.1

