mesh_plot() needs to handle 'vertices', 'faces' or 'edges' meshColor

[mdsumner]

this affects palr atm because was verifying with mesh_plot() - will verify with plot3d() until this is sorted

AustralianAntarcticDivision/palr#8

mesh <- as.mesh3d(volcano)
 mesh$meshColor <- "vertices"
 mesh_plot(mesh)
 mesh$material$color <- colourvalues::colour_values(mesh$vb[3, ])
 mesh_plot(mesh)