specify transform centre for compositor transform/scale/rotate nodes

When using the various transform nodes to scale and rotate vfx elements, it would be extremely useful to be able to specify a transform centre to scale/rotate around.

Some nodes have a draggable crosshair, such as the sun beams node, that would be ideal to have on the transform nodes too.

Currently scaling results in the element moving from it's original position, because the transform origin is the top left corner.