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