Two small improvements to the OBJ plugin

[nicolaspopravka]

Description of Change(s)

The proposed changes slightly improve the example plugin to convert between the OBJ file format and USD, and hopefully make it even more useful ;-)

Fixes Issue(s)

• OBJ files with no g statements, or a g statement not followed by a group name, could not be referenced in. With this change, one is now able to do so, like this:

def Mesh "default" (
    prepend references = @mesh.obj@</default>
 )
 {
 }

• OBJ files with vt statements could not be rendered with textures when referenced in. This is fixed.

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

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7777

[nicolaspopravka]

@sunyab I see you've filed this PR as an internal issue. What is generally to be expected from this ? Thanks.

[sunyab]

Hi @nicolaspopravka, the internal issue number is just for bug tracking purposes on our side and indicates that we've queued this PR for triage and prioritization.