Increase fill_extrusion base/height precision.

[ChrisLoer]

Fixes issue #7247: heights over 65,536 meters don't render on some devices (tested against iOS Safari). We were using the lowp precision qualifier for the fill-extrusion-base and fill-extrusion-height attributes. GLSL allows implementations to use really low precision (and small range) for lowp floats, although most implementations don't go down to the limits.

See discussion in issue #2096: as a rule of thumb we want to standardize on using highp precision qualifiers in our shaders unless there's a specific reason we know (1) low precision won't break anything we depend on, and (2) low precision is likely to be a useful performance optimization.

I tested this against a simple "landcover" extrusion layer set to 70,000 meters on iOS 11 Safari running on an iPhone 8.

Before:

After:

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality (Failing test depends on a hardware configuration we can't do on CI)
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

/cc @mollymerp @mourner @mzdraper

[ChrisLoer]

Running on Desktop Chrome:

Running on iPhone 8:

[ryanhamley]

Does this also close #7104 or is that a separate problem? It seems like it should. John tried to fix that with https://github.com/mapbox/mapbox-gl-js/compare/extrusion-mediump but it wasn't successful. It might have to be highp for this use case.

[ChrisLoer]

Yeah looks like the same problem -- it's an extrusion layer set to 100,000 meters... and I just now see that John actually had a branch that did almost the same thing as what I did here, except he went to mediump instead of all the way to highp.

[mollymerp]

📏👍