fallback for other JSON Material attributes

[goretkin]

Not exactly an issue, but I needed to visualize some wireframes, and I wanted to try using MeshCat.jl. Here's the hack I was able to do:

import MeshCat
const mc = MeshCat

"""low-level access to three js JSON interface
See https://github.com/mrdoob/three.js/blob/master/src/loaders/MaterialLoader.js"""
mutable struct OverlayMaterial{T<:mc.AbstractMaterial, OVT} <: mc.AbstractMaterial
  underlying::T
  overlay::Dict{String, OVT}
end

function mc.lower(material::OverlayMaterial)
  merge(mc.lower(material.underlying), material.overlay)
end

wireframe_material = OverlayMaterial(
    mc.MeshPhongMaterial(color=mc.RGBA{Float32}(0.8, 0.8, 0.8, 1)),
    Dict{String, Any}("wireframe"=>true))

That might be a good escape hatch to support any possible feature of the three js loaders.