Do multiple UV channels work with imported USD files?

Hi Cyril,

This does not match up with what I see in Painter.  While it's true that you cannot paint using multiple UV sets, custom shaders can in fact read multiple UV sets from a model and use them to display additional texturesets, like in the case of a layered shader.  The other texture coordinates are available via the shader inputs, specifically the input.multi_tex_coord parameter as noted in the V2F input type definition section of the documentation here: 
https://helpx.adobe.com/substance-3d-painter/scripting-and-development/api-reference/shader-api/shaders-shader-api/surface-shader-shader-api.html

So the issue is not that the second UV set is not supported in the shader, it is that the data does not exist in models imported from USD.  I tested importing the example FBX file shipped with Painter cube5uvsets.fbx, and the additional UV sets do work with a shader that uses them.  If I convert that same file to USD and import it into Painter, all the UV sets beyond the first one appear to be discarded though, and no UV data is stored in them.  This is preventing us from having parity between our USD pipeline and our FBX pipeline, which is a problem for us as we are trying to move towards everything being in USD.

-Matt