Add VTK_USE_LEGACY_DEPTH_PEELING env var handling.

Some drivers seem to have bugs that prevent the dual depth peeling from
functioning properly. Add an environment variable that will force the
renderer to use the old implementation.

Rendering/OpenGL2/vtkOpenGLRenderer.cxx

@@ -50,6 +50,7 @@ PURPOSE.  See the above copyright notice for more information.
 
 #include <cmath>
 #include <cassert>
+#include <cstdlib>
 #include <list>
 #include <string>
 
@@ -319,6 +320,19 @@ void vtkOpenGLRenderer::DeviceRenderTranslucentPolygonalGeometry()
           }
         }
 
+      // The old implemention can be force by defining the environment var
+      // "VTK_USE_LEGACY_DEPTH_PEELING":
+      if (dualDepthPeelingSupported)
+        {
+        const char *forceLegacy = getenv("VTK_USE_LEGACY_DEPTH_PEELING");
+        if (forceLegacy)
+          {
+          vtkDebugMacro("Disabling dual depth peeling -- "
+                        "VTK_USE_LEGACY_DEPTH_PEELING defined in environment.");
+          dualDepthPeelingSupported = false;
+          }
+        }
+
       if (dualDepthPeelingSupported)
         {
 #if GL_ES_VERSION_2_0 != 1 // vtkDualDepthPeelingPass is not built on ES2