This starts bringing rendering closer to how its done in -native.

Lines are added to a single vertex and element buffer. If the number of
vertices exceeds the maximum number, instead of creating a new buffer it
creates a new element group, and stores the offset of that group into
the buffer.

break geojson, but fix build

Some buckets may need to download resources (like glyphs) before begin
parsed. These are now all started first so that a previous bucket
doesn't need to be finished before starting to donwnload the next
bucket's glyphs.

Also, placement only matters for buckets with placement. Buckets that
don't have placement (lines, fills) don't need to wait until the
previous bucket finishes.

Basically:
- start downloads stuff as early as possible
- don't enforce order where it isn't necessary