Add unicode support to vtkTextRenderer.

MathText doesn't currently support UTF16 and will convert to UTF8
before rendering, but this change will allow vtkTextRenderer to be
used with the Context2D framework.

Change-Id: If798bc40d14e8136f38d6f5d19919c4328e547b4
Add a vtkTextRendererStringToImage implementation.

This is used by the Context2D framework.

Change-Id: I10cd3575e268564a5c3bb16d79cfa89a08eed3fe
Merge topic 'Context2D_TextRenderer' into master

21b3d7aa Add a vtkTextRendererStringToImage implementation.
3b807281 Add unicode support to vtkTextRenderer.