Y-Up support

[drwave]

SketchUp is natively Z-up, and USD allows one to encode a scene either Y-Up (like, say Maya) or Z-Up (like SketchUp).

The way USD works is that only the orientation of the top level layer counts, so if one has an all Y-Up pipeline but wants to get some USD assets in from SketchUp, it would be useful if there was a toggle to allow the asset that gets generated to be Y-Up.

It's not clear what the right way to do this is, though. We could rotate the scene -90 in X and then tag it Y-Up, we could transform all the points, etc.