Using get value producing attrs when checking texture assets in NormalMapTextureChecker

[sirpalee]

Description of Change(s)

The change modifies NormalMapTextureChecker to use GetValueProducingAttributes instead of querying the value of the asset parameters on the texture. This way, the NormalMapTextureChecker works better with material networks that expose asset parameters on the Material and connect them to shader parameters.

Questions:

• I'm checking the namespace of the producing attribute to see if it is an input parameter. Is there a better way to do this?
• What should we do about asset attributes produced by output parameters? Is that something we should allow, or generally, that's not expected to work? Could we configure the normal map texture checker to turn this feature on/off?

@asluk

Fixes Issue(s)

No issue was created.

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[asluk]

Looks good to me; thanks @sirpalee ! (I don't have approval perms though)

[tallytalwar]

Filed as internal issue #USD-9116