This patch consolidates the existing vtkTextMapper implementation
into the vtkOpenGLFreeTypeTextMapper class (aside from a few bits
of public API), and makes vtkTextMapper a concrete implementation
that uses the vtkTextRenderer interface. This allows newer
functionality, such as unicode and MathText, to be used from
vtkTextMappers.

Existing code will not benefit from the change as long as the
vtkRenderingFreeTypeOpenGL library is linked -- in such cases
the old implementation will be constructed by the object factory
and used.

To use the newer implementation, do not link the above library, or
remove the override it provides using the pattern shown at the start
of the new TestFreeTypeTextMapper tests.

Change-Id: I5ce46d76acf135521b95a749cca6f11e02005801