InstancedMesh: use renderable instanceMatrix default

[CodyJasonBennett]

Description

This PR populates InstanceMesh.instanceMatrix with identity matrices so it is renderable by default.

It's a common gotcha I see with new users to render an untransformed InstanceMesh and see nothing onscreen because instanceMatrix is zeroed by default.

[CodyJasonBennett]

As an aside, should instanceColor be lazily init with ones or white instead of zeroes? I'm not very familiar with the vertex-colors workflow in three, but that at least seems closer to Material.color.

[Mugen87]

As an aside, should instanceColor be lazily init with ones or white instead of zeroes?

Not sure this is necessary since the color attribute is only defined when the user actually defines an instance color. I would assume colors for all instances are define in this situation so I don't think we need to catch this use case.