Support both OpenGL and Vulkan Hgi from single build. #2564
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Description of Change(s)
Makes it possible to use hgiOpenGL or hgiVulkan from a single USD distribution, with the choice to use which happening at runtime.
Fixes Issue(s)
Currently only a single OpenGL, Vulkan, or Metal backend is built with USD. While the exclusionary nature of OpenGL/Metal makes sense for macOS, this is less true for Window or Linux and means an app cannot choose which to use at runtime in addition to making updating a render-delegate to be aware of hgiVulkan a bit more difficult than it needs to be.
Additionally, with proposals such as Autodesk DirectX backend, it makes sense to think of these interop plug-ins as a matrix where multiple backends can transfer to one-another as needed.