fixing billboardMode mode for instanced meshes

[Bastl34]

Hi.
This PR is fixing an issue with the billboardMode for instanced objects.
It was sometimes undefined which was causing the instancedMesh to use the rotation of the source transformation.

Here is a simple demo of this issue:
https://playground.babylonjs.com/#9NN1BK#1

If you uncomment the "FIX" it's working as expected (only the source mesh is rotating)

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[azure-pipelines]

Snapshot stored with reference name:
refs/pull/13265/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/13265/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/13265/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/13265/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/13265/merge
https://gui.babylonjs.com/?snapshot=refs/pull/13265/merge
https://nme.babylonjs.com/?snapshot=refs/pull/13265/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/13265/merge#BCU1XR#0

[sebavan]

Thanks a lot :-)