Add/remove mesh instance

[LeXXik]

Fixes #6680

Adds new API:

const mesh = Mesh.fromGeometry(app.graphicsDevice, new BoxGeometry());
const instance = new MeshInstance(mesh, new StandardMaterial());

entity.render.addMeshInstance(instance);
entity.render.removeMeshInstance(instance);

Tests included.

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[Maksims]

This is very useful! We had cases where we needed to do that, we had to slice the meshInstances array, then modify it and assign again, but that leads to unnecessary re-initializations and layers stuff, is just not efficient to do at runtime as it could be.
In this PR there is no unnecessary work, which is great.