Rerun the back buffer test if front buffer test fails.

Since the FrontBuffer option was taken out, the testing framework
will first test the backbuffer, and if that fails, then test the front
buffer.

However, if a test fails, the images uploaded to CDash show the wrong
data. For instance:

https://open.cdash.org/testDetails.php?test=373277492&build=4009262

shows the valid image as completely black, while the actual failure:

https://open.cdash.org/testDetails.php?test=374725352&build=4016849

shows that the number of labels displayed has changed.

Rendering/Label/Testing/Data/Baseline/TestLabelPlacer.png.md5

-6ffe5ff5814929cec5ac89bccce67352
+f772e3804b50551463c751df1b7767f3

Rendering/Label/Testing/Data/Baseline/TestLabelPlacer2D.png.md5

-95bc74d792d762d3fa4ba49b5fd89fe2
+9933986344c7db276c5bbbbefcec29dd

Testing/Rendering/vtkTesting.cxx

@@ -408,7 +408,18 @@ int vtkTesting::RegressionTest(double thresh, ostream &os)
       rtW2if->ReadFrontBufferOn();
       rtW2if->Update();
       res = this->RegressionTest(rtW2if.Get(), thresh, out2);
-      os << out2.str();
+      // If both tests fail, rerun the backbuffer tests to recreate the test
+      // image. Otherwise an incorrect image will be uploaded to CDash.
+      if (res == PASSED)
+        {
+        os << out2.str();
+        }
+      else
+        {
+        rtW2if->ReadFrontBufferOff();
+        rtW2if->Update();
+        return this->RegressionTest(rtW2if.Get(), thresh, os);
+        }
     }
   else
     {