Allow optional material configuration for the plugin

[heretique]

Hey team,

while using the Spine plugin on one of our projects we've noticed that the Spine animations were white. We discovered that that was due to the ambient light that was set to white. We fixed that issue by patching the plugin and inside createMaterial method we disabled the diffuse color

material.diffuse = new pc.Color(0, 0, 0, 0); // include diffuse component, this allows lights contribution

I think this is kind of "hacky" on our part and was wondering if there could be a way of setting this as an option on the SpineComponent?

Thanks!

[mvaligursky]

Another issues I've noticed is that the internally created material by default enables lighting - so that the meshes can be lit if required. This causes the fragment shader to be more expensive than needed, if the user does not need lighting. This is especially true for when the clustered lights are used.