MathNode: Fix typo in STEP

[wmcmurray]

While creating a MathNode like this:

const nodeC = new Nodes.MathNode(Nodes.MathNode.STEP, nodeA, nodeB);

The resulting GLSL code was something like this:

nodeVar2 = step( nodeVar1, nodeVar1 );

when it should instead be:

nodeVar2 = step( nodeVar0, nodeVar1 );

cc @sunag

[sunag]

Thanks!