Pivot each piece to face the camera

[felixmariotto]

When pieces are laid out in grid, it would be much better if they were also pivoted so that their front face look at the camera.

Currently half the pieces look away from the camera, that's not ideal :

It should happen here :

art-salad/src/puzzles/puzzleManager.js

Lines 102 to 110 in 0ec2d75

	/*
	Ideally here is would be nice to add a function to pivot the part so it doesn't look "away"
	from the center of the scene.
	Idea: computing the normal of 10 random triangle in the mesh, computing the average, and pivot
	the part so this averaged normal look the camera.
	*/

[felixmariotto]

done