iOS line shader issue

[incanus]

I seem to recall something like this with the Ejecta approach, too -- @ansis do you remember?

[incanus]

I seem to recall it being something around degenerate triangles not drawing properly, causing connections between line segments in the draw.

[incanus]

@kkaefer remembered this -- it's related to using mediump instead of highp in the shaders. We likely want to reevaluate but that's a quick fix.

[ansis]

Due to the precision differences, this was failing: https://github.com/mapbox/llmr-native/blob/master/src/shader/line.vertex.glsl#L48

The special value/comparison should be changed to something that works with mediump on all platforms.

[incanus]

Holding on this. @kkaefer is working on getting rid of degenerate triangles.

[incanus]

Closing in favor of #44.