Vertex objects without nans are all transparent

[mvdoc]

#612 fixed a bunch of bugs but introduced a new one. Pure vertex objects without any nans are displayed all transparent in the webgl viewer. The root cause is likely the nanmask/alpha mask property being filled with nans rather than ones.

MRE:

import cortex
vtx = cortex.Vertex.random("S1")
cortex.webgl.show(vtx)