Improve the nodes' drop shadow by making it scale with the view
and replace the loop for the alpha calculation with something more
explicit.
The amount of drop shadow softness was scaled with the zoom level
and therefore had a fixed screen space size. DPI and UI scale
weren't taken into account either. This patch fixes both issues by
basing the shadow softness on the `widget_unit` that scales correctly
in zoomable views and takes UI scale etc. into account.
Differential Revision: https://developer.blender.org/D13356
ed4b032155