HdSceneIndexAdapterSceneDelegate::GetMaterialResource() returns wrong network in certain situations

[robp-sidefx]

The specific alignment of the stars is as follows:

• MyRenderDelegate::GetMaterialRenderContexts() returns ["foo", "bar"]
• USD Material has outputs:bar:surface and outputs:surface

The issue:

I would expect that HdSceneIndexAdapterSceneDelegate::GetMaterialResource() would return the "bar" network but, instead, it returns the "" network.

The cause (as best I can tell):

When we call GetMaterialResource(), it iterates through our delegate's list of supported contexts, starting with "foo", and calls HdMaterialSchema::GetMaterialNetwork("foo"). This context doesn't exist in the USD file, so GetMaterialNetwork() instead attempts to return a network for the "" context which, in this case, does exits. GetMaterialResource() recognises that a valid network has been returned and immediately sends it back to us.

[jesschimein]

Filed as internal issue #USD-10102