vtkTextActor changes:
* Replace this->vtkActor2D:: with this->Superclass::
* Abstract image renderering and bounding box calculation to protected
  virtuals
* Copy Input string during shallow copy.
* Implement GetBounds()
* Add Input to PrintSelf output

vtkMathTextActor changes:
* Add a FallbackText member that will be rendered by FreeTypeUtilities
  if MathText support is unavailable.
* Removed a large amount of code that was copied/reimplemented from
  vtkTextActor.
* Implement new virtuals from vtkTextActor that render MathText if
  possible, otherwise render fallback text.

Change-Id: Ibb7aa51ec46e7778b9535dd62a00d7ef9395cd8c