Implement an XShmGetImage-based backend

[jholveck]

Changes proposed in this PR

This is close to complete, but there's a few things that need to be chased down: notably, the test_thread_safety test is failing, for some reason. It also currently doesn't work correctly if the root window size increases.

That said, this is quite promising: on my computer, the new backend can take 4k screenshots at 30-34 fps, while the XGetImage backend could only run at 11-14 fps.

• Fixes #96

It is very important to keep up to date tests and documentation.

• [x] Tests added/updated
• [ ] Documentation updated

Is your code right?

• [x] ./check.sh passed