Remove the MathTextActor classes.

* Converted tests to use regular TextActors.
* Fixed the MathText path generator to rotate paths if needed.
* Update the GL2PS text export code to check for mathtext
  strings, since mathtext/freetype are both handled by the same
  actor.
* Made vtkTextRenderer::DetectBackend public so we can peek at
  the backend from the export code.

Change-Id: I52ba3c1e916c70dd80b67d59b611db1becb0846c
Merge topic 'remove-mathtextactors' into master

d978780f Remove the MathTextActor classes.