MaterialX v1.39.4 fixes an undefined variable

[jljusten]

MaterialX v1.39.4 contains this fix:

• Fix edge case in mx_sheen_bsdf AcademySoftwareFoundation/MaterialX#2280

For this issue:

• Used of uninitialized dirAlbedo in mx_sheen_bsdf in GLSL AcademySoftwareFoundation/MaterialX#2275

Can OpenUSD update MaterialX to v1.39.4 to include this fix?

[jesschimein]

Filed as internal issue #USD-11666

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