Blender Git Loki

MaskEditor: draw active layer on top

Instead of drawing the mask layers in the sequence of their occurence, draw the active mask *always* on top.

Implementation:
- move drawing loop for splines to separate static function
- draw active mask last

Example: lowest layer is active, yet still drawn on top.
{F12140355}

This is part of an effort to make mask editing more intuitive & easy to use: https://developer.blender.org/T93097

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D13372