Add XComposite support for GNU/Linux

[jholveck]

The XComposite extension is the fastest general-purpose screen capture mechanism for X Windows, by far.

This mechanism only operates on a single window, but it operates directly in the compositing path. It’s also the only general-purpose mechanism that allows you to capture directly to a GPU buffer.

The performance difference, however, is extreme: depending on the application being captured, I got about 500 - 1000 fps for a 4k capture! While it's really only particularly helpful for capturing to a GPU buffer, for a user streaming a single window to video, the difference in performance is huge.

Also relevant: #194 (Capture single window) and #422 (Option to capture to GPU memory).