Implement a simple painting system #6752
Description
Summary
As an artist it would be great to have a live-paint system directly within gaffer in 2D and 3D contexts.
User story
What
There are multiple use cases where this would come in handy. Some of which could include creating simple masks on geometry for shading, creating primvars to control scattering, creating simple texture masks, etc. For most cases a simple primvar painter would most likely be enough, however the ability to paint on arbitrary uvs directly could also potentially come in handy.
The paint system does not need to be elaborate with multiple brush options, etc. The idea would not be to replace a full paint software like Mari, but rather give the user the ability to quickly create simple masks directly in Gaffer without having to switch to a different software.
Why
When creating masks for any of the described scenarios above that are not procedural, the user currently has to leave gaffer, bake masks in various ways and then re-import them into Gaffer. Having a simple painting system directly within Gaffer would speed up the process a lot.