BUG: Use of uninitialised value of size 8

This valgrind defect has been present for years.

There are instances where vtkDoubleScreenEdge::Case is not defined
before RasterizeTriangle. The logic in vtkSimpleScreenEdge::Init is
complex. Perhaps in the future, a more more extensive examination will
uncover the real problem. For now, the patch fixes the valgrind defect
and still passes the regression test.

Change-Id: If8a9846d1a4561447d39078daa70c5acebd8f394

VolumeRendering/vtkUnstructuredGridVolumeZSweepMapper.cxx

@@ -250,7 +250,8 @@ enum
   VTK_CASE_VERTICAL_IN_TO_OUT, // with edge equation
   VTK_CASE_VERTICAL_OUT_TO_IN,
   VTK_CASE_HORIZONTAL_IN_TO_OUT,
-  VTK_CASE_HORIZONTAL_OUT_TO_IN
+  VTK_CASE_HORIZONTAL_OUT_TO_IN,
+  VTK_CASE_UNDEFINED
 };
 
 // We use an edge equation as described in:
@@ -277,6 +278,10 @@ class vtkSimpleScreenEdge
   : public vtkScreenEdge
 {
 public:
+  vtkSimpleScreenEdge()
+  {
+    this->Case = VTK_CASE_UNDEFINED;
+  }
   // Initialize the edge by the vertices v0 and v2 (ordered in y)
   // `onRight' is true if the edge in on the right side of the triangle.
   void Init(vtkVertexEntry *v0,
@@ -1101,6 +1106,8 @@ public:
             this->Zview+=this->ZStep;
             }
           break;
+        default:
+          vtkGenericWarningMacro(<< "Undefined edge case");
         }
     }