Restore 2D Masking in 4.0 regression

[markdibarry]

This has been discussed a bit, but with Beta's feature-freeze getting closer and unless we can come up with a good proposal, we may end up with 4.0 being a downgrade in terms of 2D rendering, so it's worth having an official thread.

Currently, in 3.x, Light2D has a Mask mode that (while limited, difficult to use, and requiring specially prepared images) allows for canvas items to use light mask layers to mask other canvas items.

In the 4.0 master branch, Light2D has been replaced with PointLight2D and has the Mask mode removed. The reason for this was to separate the masking capability to a new node CanvasGroup, which would partially replace some of the features Light2D's mask mode provided. While CanvasGroup is still not yet working, a proposal for an easier and more fully featured node has been suggested. Even though this would be a welcome alternative to CanvasGroup, and remove the hard requirement of needing specially prepared images, this still would not restore the lost features provided in 3.x's Light2D. To achieve the same result as above, 4.0 requires separation of the masks and their sources, spread across the scene tree, needing the overlay AND the light source masks to be in the same CanvasGroup, shown below.

This forces much more scripting (vs none) for parent scenes to manage lighting and add/removal of the "lights" and light sources, additional RemoteTransform2Ds on the sources, and dependency on brittle, carefully crafted scene tree structures.

The purpose of this discussion is to find an implementation that would satisfy the condition of restoring the ability to mask without requiring separation of the sources and the source masks and coupling of the source masks and the target. Additionally, if the masking worked with the expected "mix" rather than "multiply" or multiple mode options, it would be ideal.

Any suggestions?