not_implemented selection into a feature

[sotaroikeda]

Gecko webrender does not use offscreen_gl_context. But it is used by webrender and requests to link to a gl library. It is nice if could remove the link in gecko webrender use case.

[emilio]

Yeah, I think I can figure something out. Alternatively, maybe making WR build without this crate is also desirable? If so, I can probably look into it (I'm the one that integrated those pieces into WR, and I think I could factor most of them into a self-contained, feature-gated place).

[sotaroikeda]

Yes, It is also nice if we could build WR without this crate.

[emilio]

See servo/webrender#773

[emilio]

This was fixed long ago.