Prefer fonts with extended characters in FontConfig lookups.

This patch modifies vtkFontConfigFreeTypeTools to return only
fonts that support, at minimum, greek character sets.

Change-Id: I29deb9223df5abbf89502eca643ac51027542cba
Merge topic 'font-config-unicode' into master

52e0f169 Prefer fonts with extended characters in FontConfig lookups.