It seems that some Indic fonts assign 'mark' glyph class to combining
spacing marks (spacing not non spacing) so my reliance on the glyph
class to set the IS_DIACRITIC flags broke those fonts. This is a bandaid
to get around the issue, plus some long rant! (at this rate, I'll be
writing "The VCL haters handbook" pretty soon).

Change-Id: I3ff892acf746d50182573f94e7e8c3c6f9464ae0