Reduce the number of ArrayBuffer objects created/used

Nobody has claimed this yet.

The analysis in https://github.com/mapbox/mapbox-gl-js/issues/4695#issuecomment-323087798 points out that we create a rather large number of ArrayBuffers. This seems to amplify an underlying memory leak in Safari, but also more generally, it's possible that we could achieve some performance gains by reducing the total number of ArrayBuffer objects we create. We could do this by:
- using an object pool
- restructuring buffers to be shared across style layers

The former is a much less invasive change, but it's also limited: the minimum number of ArrayBuffers would still be proportional to the number of active style layers. In the latter case, that would not be the case.

Start with the analysis in issue #4695, especially comment 323087798, to understand where the ArrayBuffers are created and how Safari's memory behavior is affected. Compare the object-pool and shared-buffer approaches described here; done means reducing ArrayBuffer creation without losing the expected map behavior and demonstrating the performance or memory benefit.

Written by the indexing model from the issue text.