Fix performance issue when placing many symbols on terrain

[mourner]

Exploring profiling results with @stepankuzmin, we found that an abnormal amount of CPU time was spent fetching terrain elevation values when placing symbols on terrain, and there was a lot of GC related to it as well. This PR attempts to fix the issue by avoiding creating a typed array view within DemData get, a method on a very hot path.

This also required some minor refactoring so that the class with the corresponding changes can be properly transferred from the worker to the main thread (now we hold on to a Uint8Array view rather than Uint32Array, because we can't transfer with both references).

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry for inclusion in the mapbox-gl-js changelog: (in title)

[mourner]

The rendering-3d benchmark results are indeterminate, but perhaps they don't cover the fringe cases that we've seen that are affected by this PR. https://sites.mapbox.com/benchmap-js-staging2-site/runs/448