Switch from physical to logical segmentation of buffers

[jfirebaugh]

Fixes #207.

And while here, switch to indexed rendering for collision boxes. Fixes #2038.

Physical segmentation: each vertex buffer holds a maximum of 2¹⁶-1 vertices. When a layer requires more than that, create a new vertex buffer and index buffer.

Logical segmentation: no limit to the size of a vertex buffer. When reaching 2¹⁶-1 vertices, record the vertex and primitive offsets, then reset the indexing, starting a new "segment". Use the offsets when rendering each segment.

Logical segmentation has the following advantages:

• It uses a smaller number of larger buffer objects. In theory this is more efficient for WebGL/OpenGL. However in this case the performance difference is likely negligible. We already use VAOs, which largely eliminate the cost of switching between buffers, which leaves the cost of creating them. And buffers larger than 2¹⁶ vertices are relatively rare.
• It's what gl-native does. Having the two codebases work the same way is an advantage, as we frequently need to port changes back and forth.
• It simplifies populatePaintArrays. With physical segmentation, populatePaintArrays needs to track both a buffer index and an index into the first buffer, and might need to populate multiple buffer segments. With logical segmentation, it just fills the single paint vertex buffer until it's the same length as the layout vertex buffer.

benchmark	master e03a7b1	logical-buffer-segmentation 59f39b7
map-load	150 ms	104 ms
style-load	100 ms	104 ms
buffer	1,030 ms	993 ms
fps	60 fps	60 fps
frame-duration	7.3 ms, 1% > 16ms	7.3 ms, 1% > 16ms
query-point	1.05 ms	1.16 ms
query-box	84.95 ms	88.68 ms
geojson-setdata-small	8 ms	10 ms
geojson-setdata-large	117 ms	99 ms

Launch Checklist

• briefly describe the changes in this PR
• post benchmark scores
• manually test the debug page

[lucaswoj]

The changes to ArrayGroup and Bucket look 👍. I didn't read the Bucket subclasses very closely. Let me know if you'd like a more careful review of those.

[lucaswoj]

new ElementArrayType(3) looks correct to me. I wonder why this was 1 before.

[jfirebaugh]

It worked with 1 before because FillBucket#addFeature emplaced one index at a time. Now it emplaces triangles, for consistency with FillExtrudeBucket (and native).