Optimizing performance of GreasedLineMesh._setPoints

[roland]

This PR unfortunatelly introduced a bug:

Babylon.js Playground

        points: [
            [
                -1,1,0,
                1,1,0
            ],
            [
                -1,-1,0,
                1,-1,0
            ]
        ]

Produces:

Expected:

Can you check it please?