Encapsulate SymbolBucket's unique requirements

[jfirebaugh]

SymbolBucket is in several ways the "odd bucket out":

• It's the only bucket type that works with multiple program interfaces -- one for glyphs, one for icons, and one for collision boxes.
• It's the only bucket type whose instances are reused to build buffers multiple times.

Previously, the Bucket base class was generalized such that it could support multiple program interfaces for any Bucket subtype, and re-create buffers at any time. However, this added complexity to all subtypes other than Symbol.

Now, SymbolBucket encapsulates its differences:

• It uses ArrayGroup/BufferGroup directly, creating one for each of its three interfaces. In turn, the other bucket types, their corresponding layer rendering functions, and the Bucket base class, can all assume a single program interface.
• It recreates these arrays when needed, while the Bucket base class creates them once, in the constructor.

[jfirebaugh]

This is crashing the buffer benchmarks for some reason.

[jfirebaugh]

I made a 🐛 . Fixed.

benchmark	master 965a2f3	symbol-bucket f74e58c
map-load	144 ms	92 ms
style-load	118 ms	117 ms
buffer	1,053 ms	1,078 ms
fps	60 fps	60 fps
frame-duration	7 ms, 0% > 16ms	6.9 ms, 0% > 16ms
query-point	1.10 ms	1.14 ms
query-box	66.95 ms	67.50 ms
geojson-setdata-small	9 ms	8 ms
geojson-setdata-large	275 ms	278 ms