Switch to using alphabetic baseline for local glyphs.

[ChrisLoer]

This PR integrates the TinySDF changes at mapbox/tiny-sdf#30 which switch to drawing glyphs on the "alphabetic" baseline instead of the "middle" baseline. This gives better visual alignment between fonts. The new version of TinySDF will have to be published before this PR builds successfully.

Here's an overlay of the Meiryo and YaHei fonts, showing how the alphabetic baseline matches expectations (Meiryo "broke" earlier assumption because it has a very large font descender):

TinySDF's "top" metric now means "distance from alphabetic baseline to top of glyph" which seems pretty intuitive but notably different from what https://github.com/mapbox/sdf-glyph-foundry generates. This PR adjusts to bring the two metrics mostly into line.

Here are some cross browser/font/platform test results (it would be great to figure out a good way to automate this):

Chrome MacOS Meiryo : Server Arial

Chrome macOS Meiryo:Meiryo

Chrome MacOS san-serif all

Chrome MacOS sans-serif: server Arial

Chrome Windows 10 : sans-serif all

Chrome Windows 10 sans-serif : Server Arial

Edge Windows 10 sans-serif : Server Arial

Edge Windows 10 sans-serif all

Firefox MacOS san-serif : Server Arial

Firefox MacOS sans-serif all

Firefox Windows 10 sans-serif : Server Arial Unicode

Firefox Windows 10 sans-serif all

For an example of what was broken before, see this customer screenshot using Meiryo / Server Arial:

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog:

<changelog>Switch to using alphabetic baseline for locally-rendered glyphs to avoid misalignment on fonts with large ascenders/descenders.</changelog>

@mourner @asheemmamoowala @tsuz

[ChrisLoer]

I didn't re-run my manual TinySDF performance benchmarks, but I did a sanity check on a glyph-heavy Tokyo scene to rule out major perf regressions on Chrome or Firefox. There's no particular reason to expect this to be a performance impacting change (the size of the glyph and the canvas is still the same, just drawing at different positions and returning a different "top" metric.

As for changelog, I'm not sure this needs a changelog? We could create a public-facing issue about the baseline discrepancy for Meiryo, and record this as a fix for that issue. I don't think this really counts as a whole new feature, more just a tweak to a recently-released feature.

[mourner]

@ChrisLoer we don't have to make an issue for every changelog entry — I think it's OK to just say something like "Fix an edge case with glyph alignment when using local font rendering with certain fonts".