Handle UTF-8 strings in vtkMathTextFreeTypeTextRenderer.

Treat incoming strings as UTF-8 and add tests on extended character
sets.

Change-Id: Id9c51012b263c17869073b4b72057e54d57fd07b
Merge topic 'unicode-textrenderer' into master

edf5bcfa Handle UTF-8 strings in vtkMathTextFreeTypeTextRenderer.