NF display values at vertex in webgl viewer

[mvdoc]

This PR adds the feature of visualizing the VertexData value when hovering on the surface or when clicking on a specific vertex.

Example code to test that it works as intended (unfortunately it's hard to automate):

import cortex
roi = cortex.get_roi_verts("S1", roi=["EBA"], mask=True)["EBA"]
roi = roi.astype(float)
vtx = cortex.Vertex(roi, "S1")
cortex.webgl.show(vtx)

Example images