Morph weights should be affected by the anim layer weight

- it seems that when the animation modifying morph weights is weighted down, the morph weights are unaffected, but should be weighted down.

see more here + repro
https://forum.playcanvas.com/t/shape-keys-blend-weight/40764

I forked the project here to avoid it possibly being deleted: https://playcanvas.com/project/1392804/overview/shape-key-weights-fork