InstancedMesh raycast fix

[gkjohnson]

Fix #671
Based on #673

cc @agargaro I made some of the remaining changes requested from #673 - let me know how it looks and I'll merge and publish!

[agargaro]

Just a small consideration:

without using a scene bvh, a user usually needs to iterate over all the meshes in the scene to check which one is intersecting the mouse, this would mean calling the raycast function (and also decompose) N times.

If a user has 50k objects, decompose takes 2ms, instead of 0.5ms with custom method.
https://stackblitz.com/edit/getworldscaleper?file=index.ts

if it's okay for you, we can go ahead with the merge (after removing those 2 var from the tests) :)

[gkjohnson]

without using a scene bvh, a user usually needs to iterate over all the meshes in the scene to check which one is intersecting the mouse, this would mean calling the raycast function (and also decompose) N times.

Thanks! Good catch. It would be nice if three.js provided this scale function internally considering there's already Matrix4.copyPosition and Matrix4.extractRotation functions, as well. Adding Matrix4.extractScale( target ) might be an easy add to three.js if you'd like to give a PR a go. But in the mean time we can keep the function here.