Free vertex/index buffers after uploading to GPU

[mourner]

Closes #11432. Array buffers weren't actually freed because we didn't also remove views that pointed to them. This PR restores the original intent of the optimization. I've launched some benchmarks on it — let's see if this makes a difference to performance.

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 inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improve GL JS memory footprint by freeing memory more eagerly after loading tiles.</changelog>

[mourner]

Looks like it does improve memory footprint very significantly. For a typical dense view of SF, the main thread in main retains 16MB of typed arrays, while in this branch, it's 7MB.

Before	After

Benchmarks (loading-full) don't show meaningful deviations in most metrics, but total_objects_in_memory shows a clear win: