[GSUB] Wire the font, not just the face, down to substitute()

[GSUB] Wire the font, not just the face, down to substitute()

We need the font for glyph lookup during GSUB pauses in Indic shaper.
Could perhaps be avoided, but at this point, we don't mean to support
separate substitute()/position() entry points (anymore), so there is
no point in not providing the font to GSUB.