Support injection of hinted metrics

[dfrg]

At least for advances. This means adding something like hb_font_funcs but we don’t need the full gamut.

[rsheeter]

It would be helpful if the issue said why we need this?

[behdad]

It would be helpful if the issue said why we need this?

Because the advance widths used in shaping should match those from the hinted glyph rasters.