Autodesk: Prevent undesired materialx garbage collection

[erikaharrison-adsk]

Description of Change(s)

Summary: Add a member variable for HdMaterialNetwork to hold the generated MaterialX Shader to prevent unnecessary garbage collection.

With the current code base, every time we re-apply an existing material, the MaterialX::ShaderPtr is still considered as "the first instance"

https://github.com/PixarAnimationStudios/OpenUSD/blob/0b18ad3f840c24eb25e16b795a5b0821cf05126e/pxr/imaging/hdSt/materialXFilter.cpp#L1008

even the underlying HdMaterialNetwork is cached on application side. The reason is that after every drawcall, there is a garbage collection process (https://github.com/PixarAnimationStudios/OpenUSD/blob/release/pxr/imaging/hdSt/resourceRegistry.cpp#L1098). The previously unapplied material (MaterialX::ShaderPtr) is garbage collected because HdStorm considered that nobody needs it anymore even though, as I said, the HdMaterialNetwork is cached. This is not the case for PreviewSurace or any other custom glslfx because there is member variable called _surfaceGfx which holds a reference of the generated glslfx thus the reference count is at least 2 and the ResourceRegistry won't garbage collect it.

To address this issue, we can simply add a member variable in HdMaterialNetwork to hold the generated MaterialX glslfx shared_ptr just like the _surfaceGfx to prevent the unnecessary garbage collection for MaterialX shader as well.

Fixes Issue(s)

• N/A

• [X] I have verified that all unit tests pass with the proposed changes
• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9573

[jesschimein]

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