Implement vtkTextMapper using vtkTextRenderer.

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
Removed the XML shaders and associated classes

These have not been used in quite some time, add dependencies on XML
in the core rendering API and have been superseded by their sequels.

Disabled GenericVertexAttributes tests for now due to use of XML
shader API, along with the materials object example.

Change-Id: I3c57c46b1d3cc7f550bd30e1abcd6e5d87347d36
vtkIOImage not needed by vtkRenderingCore now

Now that the XML shader stuff is gone the vtkIOImage library isn't
needed either.

Change-Id: I64adeabd461c5adfb3793450576460aa477bfab7
Removed vtkUtilitiesMaterialsLibrary

This is no longer needed/used.

Change-Id: I19ad4e59f654337b4cd7c125876e8323cd534454
Merge topic 'textured-textmapper' into master

8707ec0c Implement vtkTextMapper using vtkTextRenderer.
Merge topic 'rendering-remove-xml' into master

6501ce8c Removed vtkUtilitiesMaterialsLibrary
4b96f6f3 vtkIOImage not needed by vtkRenderingCore now
67cb974b Removed the XML shaders and associated classes