MeshMatcapMaterial no longer requiring a matcap.

[mrdoob]

@WestLangley

I was able to use DataTexture inside MeshMatcapMaterial. I guess the problem was that the example was trying to set matcap.image and DataTexture uses that property differently.

While looking at the issue, I realised that we could hide this complexity in the shader. If matcap is not set now the shader writes white instead. The user will have to call material.needsUpdate if they set a matcap after it has been compiled already, but that's how map, bumpMap, etc work too.