[HarfBuzz] behavior of mark-width-zeroing

[Jonathan Kew]

Hi Behdad,

Our current mark-zeroing code, in zero_mark_widths_by_gdef() and 
zero_mark_widths_by_unicode(), modifies only the advance of the glyphs, 
so that they no longer take up any space on the line.

I'm wondering whether we should also adjust the offset, by subtracting 
the advance from it before we zero the advance. (Though perhaps only if 
there's no GPOS positioning?)

In particular, this is required for correct rendering of old fonts such 
as MS Sans Serif codepage 874 (Thai), which has spacing glyphs for the 
Thai vowel/tone marks. AFAICT, what Uniscribe does with it is to zero 
their advance width *and* offset them to the left by their original 
advance, so that they overstrike the preceding glyph, even though the 
images in the font appear to the right of the origin.

WDYT? Can you think of fonts where this would *not* be appropriate?