OpenGL: move interop_sw to a dedicated module

Move the software OpenGL interop to a dedicated module, simplifying the code further ~~and allowing the interop to be picked individually.

There's some part missing in this MR to avoid changing the behaviour. In particular, the previous way was ensuring that other display are tried before falling back to memory-mapped chroma. I don't really know whether we want to keep this behaviour (and fail the software interop except when it's being forced) or automatically fallback like currently done.~~