This node outputs the direction vector, or tangent of a curve at every
control point. For poly splines this is simply the evaluated points, so
it all works very simply. For Bezier splines it takes the tamgent at the
evaluated point corresponding to each control point, and NURB splines
are interpereted as poly splines built from their control points.
It is similar to the curve parameter node: D12548
Internally the node is called "Input Tangent" to simplify using it for
mesh tangents as well in the future like the "Normal" node.
