Allow scaling of the circle used as cursor in the color widget
based on the zoom.

The small circle used to choose the hue/saturation and value in the
color widget was drawn with a fixed screen space size. With the
recent extension of the zoom limits in the node editor the lack of
scaling can lead to bizarre results.

master	patch

This could have been part of D12842 but the implementation is
pretty independent and I wasn't sure what the procedure was, since
the patch was already accepted.

Based on a fix provided by @Erik Abrahamsson (erik85)