Allow accessors to vary per vertex for path layer and polygon layer

[kylebarron]

In most cases, the user will pass one value per geometry, and then the layer will handle expanding that value to be per-vertex (arrow.Vector<arrow.Float> for example). But the layer should also allow a per-vertex input (i.e. arrow.Vector<arrow.List<arrow.Float>>). Then the user has total control over the per-vertex styling.

The layer should validate that the array offsets are the same for the passed-in vector as for the geometry offsets.