Enable GPU Raycast volume mapper for recent NVIDIA drivers on Mac OS X

GPU Raycast mapper was crashing for GL_VERSION "2.1 NVIDIA-1.6.XX".
However it works fine for tested version "2.1 NVIDIA-8.0.61"

Commit cherry-picked and adapted from 74079f9

Rendering/VolumeOpenGL/vtkOpenGLGPUVolumeRayCastMapper.cxx

@@ -2139,16 +2139,23 @@ void vtkOpenGLGPUVolumeRayCastMapper::LoadExtensions(
   this->UnsupportedRequiredExtensions =
     new vtkUnsupportedRequiredExtensionsStringStream;
 
+  const char *gl_version=reinterpret_cast<const char *>(glGetString(GL_VERSION));
   // It does not work on Apple OS X Snow Leopard with nVidia.
+  // However it works with Apple OS X Lion with nVidia
   // There is a bug in the OpenGL driver with an error in the
   // Cg compiler about an infinite loop.
 #if defined(__APPLE__) && !defined(APPLE_SNOW_LEOPARD_BUG)
 
   (void)window;
-  this->UnsupportedRequiredExtensions->Stream<<
-    " Disabled on Apple OS X Snow Leopard with nVidia.";
-  this->LoadExtensionsSucceeded=0;
-  return;
+  const char *gl_vendor=reinterpret_cast<const char *>(glGetString(GL_VENDOR));
+  const char *minNVIDIAVersion = "2.1 NVIDIA-8";
+  if(gl_vendor == 0 ||
+     (strstr(gl_vendor,"NVIDIA Corporation")!=0 &&
+      strncmp(gl_version, minNVIDIAVersion, strlen(minNVIDIAVersion)) < 0))
+    {
+    this->LoadExtensionsSucceeded=0;
+    return;
+    }
 
 #else