Vertex Color Material support for viewports

[teratron]

Need to see the effects of painting vertex colors that are used in materials in the viewport. For example painting texture blends with the vertex color channels. This is a common workflow especially for games.
Right now the user experience for working with vertex paint is fairly obtuse. For example the only way to access vertex alpha is to use an OSL UVW Channel node, then input an override value node of -2. Most people coming from Unreal are used to seeing something like this. 

I recommend updating to the vertex color node with split RGB/XYZ outputs and a drop down selector in the node for which channel you want to use. For example, I select vertex alpha from the dropdown and then can output RGB together or split them off.

Most importantly the viewports need to be able to properly display the effects of the colors when used in materials.

[dmitriy.shpilevoy]

Meanwhile you can use this DX mat to display vc texture blending in viewport:

https://dimwalker.gumroad.com/l/dwvcdxmat

[benjamin4MQAH]

I think we "just" need an OSL vertex color node. I'm trying to figure out how to write those shaders but documentation seems sparse.