* Complete wasm backend memory mgmt, extract common glow code
Similar to 3e8d2d7d, but for the wasm backend.
Also takes a pass at consolidating some of the common code between the
glow-native and glow-wasm backends. At this point, the backends just
diverge in their window handling, and the data types of their OpenGL
data types (e.g. vertex_buffer.id is WebBufferId vs. uint32 on native)
* glow impls share prerender innards
* coalesce vertex/buffer types
* minimize diff
* clarify module naming
as part of this, wound up grabbing fresh OSM and GTFS. but actually cant
reimport OSM right now, because fixes reference old OSM IDs and things
break. so sticking with old OSM, but new GTFS. urgh.
