[DISCUSS] Add `DLDeviceType` value for OpenGL buffer?

Motivation: I'm looking to add DLPack interoperability to the Perl Data Language (PDL - https://pdl.perl.org/ if interested). That currently only supports CPU-based memory, so that's fine. But PDL is about to start handling OpenGL buffers to keep computation on the GPU where possible, and it would be great to be able to describe that in DLPack terms.

Therefore, I would very much like to add a `DLDeviceType` enum value called (probably) `kDLOpenGL`. Will that be acceptable? (There may need to be two, `kDLOpenGLTexture`, `kDLOpenGLVBO`)