Add support for mouse warp using libei

[slouken]

The current method of hiding the cursor, warping, and showing the cursor can be extremely expensive on some compositors, such as gamescope. libei is a much more efficient path for warping the mouse on Wayland, so we should use that where supported.

Maybe this code should go in src/common/linux, so it can be shared between the X11 and Wayland backends?

@misyltoad, is this something you'd like to tackle? The recent XTest support is a good path to follow for this.