Docs: Clarify InstancedMesh.setMatrixAt() needs update.

[gonnavis]

Clarify "You have to set .instanceMatrix.needsUpdate flag to true after called .setMatrixAt()."
I have been troubled by this for a long time, even though I finally found it mentioned in the previous lines.

[Mugen87]

I wonder if we could hide the necessity to set the needsUpdate flag at all by introducing a private dirty flag that indicates that the buffer attribute requires an update. It would be set to true when using setMatrixAt(). The dirty flag could then be checked by the renderer per frame. If set to true, needsUpdate is set to true.

I think it's better use a separate dirty flag for this and not directly set needsUpdate in setMatrixAt(). Otherwise the internal buffer attribute version would be quickly reach high numbers.

[gonnavis]

I also think that call self method but then require to set another property's needsupdate is a bit strange.

[mrdoob]

My goal with InstancedMesh was to provide a way to render many objects at the same time with minimal CPU (and memory) usage. I thought about adding a flag in setMatrixAt() but I worried that it would increase the CPU usage when updating the matrices of thousands of objects per frame.

[mrdoob]

Can you change the text to this?

Make sure you set [page:.instanceMatrix][page:BufferAttribute.needsUpdate .needsUpdate] to true after updating all the matrices.

[gonnavis]

@mrdoob Changed.

[mrdoob]

Thanks!