Add unicode text support to charts and context2D.
This patch assumes all 8-bit strings rendering in the OpenGL context2D device are UTF-8 encoded, and uses vtkUnicodeString::from_utf8 to allow extended character sets to be used in Context2D 8-bit strings. This also fixes an issue with UTF16 rendering alignment, and updates the image cache to use 16-bit strings for freetype renderings and 8-bit strings for mathtext rendering. Change-Id: Ic5f905f0a2ba70919e5df59120ce7a8ee536945a
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