Use indexed renders for lines

[kkaefer]

We're currently using triangle strips for drawing the wide lines, but we should move to use indexed renders and GL_TRIANGLES. This allows us to avoid adding degenerate triangles over and over again, as well as benefit from vertex caching because we can share indices.

Until now, we got away with drawing triangle strips and using out of viewplane degenerate triangles. However, mobile devices are using a tiled GL renderer that penalizes that behavior because it can't confine rendering to one tile if we have geometries spread around the entire viewport.

• support overflowing vertex buffers

[kkaefer]

(This is based on #55)

[kkaefer]

Improves the situation slightly, but still not to a level where we can get good performance on difficult tiles like 6/33/21 (Netherlands):

before: 142.3ms CPU     42.8ms GPU
after: 130.7ms CPU     22.3ms GPU