title: vtkDelaunay3D::SetAlpha() does not accept values below 0
body: <p>I'm trying to use VTK's Delaunay3D() to get a minimal bounding surface on my data using the alphaShapes algorithm. The particular dataset I'm working on is generally toroidally- or cylindrically-shaped, so by my understanding I should be trying to find a value &lt; 0 for alpha. The class, however, does not seem to be able to handle negative floats. This can be confirmed by this minimal example:</p>
  <pre><code>#include &lt;iostream&gt;
  #include &lt;vtkSmartPointer&gt;
  #include &lt;vtkDelaunay3D&gt;
  
  void test() {
      vtkSmartPointer&lt;vtkDelaunay3D&gt; dataMesh = vtkSmartPointer&lt;vtkDelaunay3D&gt;::New();
      dataMesh-&gt;SetAlpha(-.1);
      std::cout &lt;&lt; dataMesh-&gt;GetAlpha() &lt;&lt; std::endl;
  }
  
  int main(void) {
      test();
  }
  
  </code></pre>
  <p>I get an output of 0, and this is reflected in visualizations of my actual data -- I get a big ugly diamond instead of a beautiful donut. If SetAlpha() is given a positive value, VTK responds as expected.</p>
  <p>Is this a known issue? Are there workarounds?</p>
  <p>SYSTEM: Ubuntu 20.04, using gcc version 9.4.0 with CMake. VTK 9.1 for c++.</p>