Add triangle predicates to pointer down and up events.

[f-rooom]

This PR adds scene.pointerDownTrianglePredicate and scene.pointerUpTrianglePredicate, which function analogous to the already implemented scene.pointerMoveTrianglePredicate. This allows to set triangle predicates for use with ActionManager or Behaviors like PointerDragBehavior, which currently is not possible.

There should be no performance impact when not setting the predicates, as is with pointerMoveTrianglePredicate.

Relevant forum post: https://forum.babylonjs.com/t/triangle-predicate-for-picking-triggers-in-actionmanager-and-pointerdragbehaviour/47761

I hope this fits all the guidelines. :)
Open for review!

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[RaananW]

LGTM. back-compat, optional :-)