Add XFixes support and CusorBarrier command.

[somiaj]

CursorBarrier is a command to create a PointerBarrier using the XFixes extension. This barrier can be used to confine mouse movement to any rectangle on the desktop. This first adds XFixes as an optional dependency, and if XFixes is included, the CursorBarrier option can be used to create PointerBarriers.

CursorBarrier can be use to both create and destroy barriers. Currently MAX_BARRIERS is set at 16, so only up to 16 barriers can be created at a single time. The use of CursorBarrier is documented in the manual page.

The main intent of this is to allow a toggle to confine the mouse to a specific monitor (often the current monitor) using CursorBarrier screen c and CursorBarrier destroy, as mentioned requested in #603. This can also be used to confine the mouse to any specified rectangle or screen on the desktop, define multiple barriers, and use CursorMove or WarpToWindow to move between them.

I almost added a window option to specify a barrier to a specific window, but opted not to at this time. The following will confine the mouse to a selected window already (the only thing a window option would provide is computing coordinates relative to the windows position and size).

Pick CursorBarrier coords $[w.x]p $[w.y]p $[math.+.$[w.x],$[w.width]]p $[math.+.$[w.y],$[w.height]]p