Adjust mark advance-width zeroing logic for Myanmar

Adjust mark advance-width zeroing logic for Myanmar

Before, we were zeroing advance width of attached marks for
non-Indic scripts, and not doing it for Indic.

We have now three different behaviors, which seem to better
reflect what Uniscribe is doing:

  - For Indic, no explicit zeroing happens whatsoever, which
    is the same as before,

  - For Myanmar, zero advance width of glyphs marked as marks
    *in GDEF*, and do that *before* applying GPOS.  This seems
    to be what the new Win8 Myanmar shaper does,

  - For everything else, zero advance width of glyphs that are
    from General_Category=Mn Unicode characters, and do so
    before applying GPOS.  This seems to be what Uniscribe does
    for Latin at least.

With these changes, positioning of all tests matches for Myanmar,
except for the glitch in Uniscribe not applying 'mark'.  See preivous
commit.