Add metal and smoothness texture maps to mrtk standard shader options

[StephenHodgson]

Overview

Currently the metal and smoothness maps need to be embedded into the albedo texture, but then that limits the ability to use transparency in the albedo alpha.

Adding the metal and smoothness texture slots will give developers greater flexibility in creating materials.

[Cameron-Micka]

This shouldn't be too hard to add, I would be worried about enabling these features on HoloLens for perf reasons. But, with IHMDs it should be fine. I'm a tad busy right now, but can add this in the coming weeks!

[jerrygg]

Has there been any progress on that front?

[StephenHodgson]

@Cameron-Micka bump. Once that's in I think we should port that feature over to vNEXT.

[Cameron-Micka]

I haven't had a chance to start this yet, but I agree I will make sure to merge all latest changes to vNEXT. I also have a few other enhancements I would like to integrate into the MRTK from our local version.

[Cameron-Micka]

I have some cycles to add these features. I thinking of approaching it as one additional channel texture. Laid out as follows (to match Unity's standards):

Red -> Metallic
Green-> Occlusion
Blue -> Emission (Unity uses 3 channels for this.)
Alpha-> Smoothness

[Cameron-Micka]

I just created a pull request for this! Implementing it via channel maps requires a bit more texture authoring time, but is a big win for performance.