Add support for second UV in one Material/Mesh

[jakubadin]

Hi!
Can we get support for second UVs in one mesh one material?
It would be a killer feature for environment artists from the game industry.
We very often use 1st UV channel for tiling texture or trim texture -> overlaping UV faces everywhere. 2nd UV channel is used for global RGB Mask texture.

Right now everyone is painting masks for buildings in substance/mixer/armory without seeing the end result, so a lot of "FromSubstanceToUnrealAndBack" is happening.
Just let artists choose which texture should use which UVs in their material.

For a better understanding, there are some useful videos:

BlendcerGuruP: Basic Princip
https://youtu.be/1aNnERnHRZg?t=407

Cario Goodbrand - whole procces
https://youtu.be/1aNnERnHRZg?t=407