[skrifa] metrics.left_side_bearing does not apply variations

[simoncozens]

For Roboto:

            let aw = glyph_metrics.advance_width(*old_gid).unwrap_or(0.0);
            let ls = glyph_metrics.left_side_bearing(*old_gid).unwrap_or(0.0);
            println!(
                "Collecting metrics for gid {:?} at location: {:?}: aw {}, ls {}",
                old_gid, location, aw, ls
            );

Collecting metrics for gid GlyphId(20) at location: LocationRef([-1.0, 0.0]): aw 1121, ls 115
Collecting metrics for gid GlyphId(20) at location: LocationRef([0.0, 0.0]): aw 1151, ls 115
Collecting metrics for gid GlyphId(20) at location: LocationRef([1.0, 0.0]): aw 1187, ls 115
Collecting metrics for gid GlyphId(20) at location: LocationRef([0.0, -1.0]): aw 1011, ls 115

Notice that advance width does get variations applied, but lsb does not.

[behdad]

LSB doesn't get variations probably because HVAR doesn't have them. The way to get them is to apply gvar variations and look at phantom point deltas. But that's ugly and expensive to do in GlyphMetrics by default IMO.

How does hb-subset deal with this?

[dfrg]

Since LSB lookup doesn’t really occur on any hot path, I suppose we could just add the gvar fallback by default.

[dfrg]

We already have the code to do it:

fontations/skrifa/src/metrics.rs

Line 343 in 03c5abf

lsb += self.metric_deltas_from_gvar(glyph_id).unwrap_or_default()[0];

Just need to add an additional check to see if lsbMappingOffset is null.