fdo#70968: Incorrect rendering of Devanagari short 'i' vowel

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
Reviewed-on: https://gerrit.libreoffice.org/6485
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>