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

Fix camera border bug in passepartout render view

{F10761402}

With active viewport render from camera view, the camera border shows up, even when passepartout and overlays are disabled.

By moving the line-drawing code to the passepartout section, it is effectively disabled when passepartout is off.

Reviewed By: sebastian_k

Differential Revision: https://developer.blender.org/D12745