Blender Git Loki

UI: Fix scaling of HSV cursor when zooming

The small circle used to choose the hue/saturation and value in the
color widget was drawn with a fixed screen space size. Now scale the
circle used as cursor in the color widget based on the zoom. This
could have been part of a9642f8d6130 but the implementation is
different.

Based on a fix provided by Erik Abrahamsson

Differential Revision: https://developer.blender.org/D13444