Support symbol stroke outline

[jonmmease]

We need to support the stroke width and color for symbol marks.

One subtly is that stroke width is constant as the symbol size varies, so we can't simply compute the stroke vertices on the CPU and then transform them in the vertex shader. lyon should be helpful here as it provides vertex normals when creating stroke tessellations. This will allow us to resize the stroke width to compensate for size scaling.

[jonmmease]

Done in #12