Blender Git Loki

GPencil: Refactor: New linked mask system

The new system is more flexible and a bit simpler from the code side.

Each layer has a mask layer slot. If filled with a correct layer name,
the mask layer will be rendered again in a separate buffer. This buffer
will then be used in the blend pass to modulate the opacity of the layer.

This approach is not 100% equivalent to the standard layering in most 2D
applications if using varying alpha on a mask applied to multiple layers.
However this technique does support any ordering arrangement of the masks.

The layers used as mask can now still render normaly.