Viewer config : Add diagnostic shading mode for usd:purpose

[johnhaddon]

Some open questions here :

1. This suffers from a problem I've just discovered that all the diagnostic modes have : they are overridden by any Cs primitive variables on an object. It feels like we should fix this, but I'm not sure where - the easiest would be to use a different shader than Constant. Should this be a new shader, in which case what should it be called or should it be private ? Or should we use the existing FacingRatio?
2. Should this automatically turn all includedPurposes on so you are visualising, or is that surprising or at the very least a performance hazard.
3. What should the colours be?

[murraystevenson]

FacingRatio would get my vote, individual objects are more easily discernible than with a Constant (or a new private ConstantButIgnoresCs) on everything. An interior scene like the ALab would be much easier to navigate with this diagnostic mode enabled if it were using FacingRatio.

I think this should probably not enable all includedPurposes. As you say, that would seem surprising at best and a performance hazard at worst.

I don't mind the colours you've picked, though an alternate could be something with a bit more of an associative quality, like "Red for render, green for guide, blue for p(b)roxy..." 🤷

[johnhaddon]

FacingRatio would get my vote

Cool - I've pushed an update that uses that.

[murraystevenson]

Looks good, thanks!