easier graph control point selection, by always grabbing closest point instead of adding a new point.

The little curve graphs on nodes like the rgb curves are extremely finicky to select unless zoomed right in. What usually happens is you think you're clicking close enough to grab it, but you accidentally add a new control point.

I suggest that to add a new control point, the user must hold the ctrl key when left clicking. Without the ctrl key pressed, the nearest control point is grabbed.