Use a shared memory buffer for the X11 backend

[notgull]

The current X11 backend sends the image over the wire, which is slow and inefficient. We should probably use the MIT-SHM strategy for this crate if it's available.

(Also, we probably shouldn't create a new image every time we run the set_buffer command, but I think we can take care of that when we solve #37)

[ids1024]

if it's available

That extension has a copyright date of 1991. Is it ever not supported by modern X servers on modern OSes?

[notgull]

I think xvfb doesn't support it, but I'd have to double check. At the very least, libcairo falls back to normal image sends if it doesn't detect MIT-SHM, but that could just be legacy code.

[ids1024]

Yeah, that could just be legacy code in cairo. Though even the maintainers of libcairo might not be sure unless they remove it and see if anyone complains.

It's good to have the fallback if it's relevant in any modern context, but probably not if it is needed only on 2-3 decade old Unix systems that aren't going to be supported anyway.

[ids1024]

Oh, I guess shared memory also wouldn't be available with a remote X server.