Add extent support. Refs #1227

[tmcw]

This works on my example style.

• Adds a cache for the typed arrays that set extents
• Fixes drawing ratio of lines to match different extents

Main point of uncertainty:

• This adds support for a single extent value per vector tile. While
  multiple values are technically permitted there aren't any tiles that
  create them yet.

Could use a review /cc @ansis @jfirebaugh @kkaefer

[jfirebaugh]

For terminology, we should reserve the word "buffer" for the GL concept and call the backing arrays "arrays" or "buffer arrays". We probably don't do this elsewhere but we should.

I don't think it's worth caching the array creation -- it'll be heavily dominated by gl.createBuffer / gl.bufferData. If you want to cache something, cache the buffer.

[tmcw]

Okay - given that caching the buffer seems pretty tricky and this seems to perform pretty well, i'll remove the cache.

[tmcw]

Removed cache in 598891b

[kkaefer]

This adds support for a single extent value per vector tile. While multiple values are technically permitted there aren't any tiles that create them yet.

Is there major effort associated with doing it per-layer? While there currently may not be vector tiles that specify multiple extents, compositing vector tiles may easily produce such tiles.

[tmcw]

@kkaefer mostly it's just extremely hard to follow how a "source layer" becomes what we actually paint and where i would attach this data to make that possible. If you can give a pointer or two in that direction it's probably not terribly hard to implement.

[tmcw]

@ansis @kkaefer Dove into multi-extent-per-tile in #1236 and figured out that it entails a much larger refactor, since we'd have to split Tile into two classes, one for the tile id and the other for buffers & transforms.

I think we should land this PR now, refactor & document gl-js, and then implement support for multiple extents in a single tile. Changed this one to Refs #1227