transferrable symbol buffer quest

[mollymerp]

This is step 1 to make cross-source label collision detection and placement possible.
Prior discussion on this topic at #1042
Plan is for me to tackle this with support from @yhahn. Thanks x 💯 to @ansis for helping think this through.

Goal:

Let information needed to do symbol placement be transferrable as an array buffer between workers + main thread.

To do this, we need to:

1. Because we can only add typed numbers to a structArrayType, we first have to convert symbolInstance.glyphQuads into a structArrayType in a manner similar to the collisionBoxArray
2. using the results from 1. convert the symbolInstances array into a structArrayType
3. instead of calling placeFeatures() in symbol_bucket.js and worker_tile.js, add this new symbolInstancesStructArray aka symbolBuffer to workerTile.js here to transfer it back to the main thread
4. make modifications to other files in js/source/ to make sure the transferred data is available where needed
5. create a process to transfer the symbol_buffer data to a new worker where it could be processed together with symbol_buffers from other sources using placeFeatures() at that time.

When that is all done, we plan on starting another quest! Thoughts on that from @yhahn:

Cross-source placement quest

Goal: get all symbol buffers for a tile to a single worker so it can manage collision for the symbol buffers all at once.

This quest is a lot more murky to me because I am not familiar with the tile pyramid and sources yet.

• Use a single tile pyramid rather than a pyramid per source
  • Why: so that the single pyramid can determine when all workers have sent back symbolbuffers for a tile to trigger a single placement event that has all the symbolbuffer information for that tile
• Sources live under the single tile pyramid and manage appropriate tile based on what the tile pyramid needs/requests (?)
• Workers' handling of messages/buffers remains the same (?)
• Main process needs to manage symbol layers differently from others, receiving symbol buffers and queuing work for the symbol placement worker

[mollymerp]

Closing here since #2642 has 🚢'd
Thoughts on cross-source placement quest 👉 #2703