Add `create_framebuffer_from_gl_name` for interop with `surfman` and similar crates.

[iamralpht]

surfman supports multicontext GL on some platforms by creating a texture shared between contexts, which one context uses as an FBO, and the other context samples from. This change to glow makes it easier to use the FBO in a context that is also doing most of its rendering using glow.

[bors[bot]]

Build succeeded:

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• build (macos)
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• build (windows)

[iamralpht]

Awesome, thanks for merging @grovesNL !