xfixes: clean up request handling

The current way of switching between Xinerama and single-screen handlers is quite complicated and needs call vector tables that are changed on the fly, which in turn makes dispatching more complicated.

Reworking this into a simple and straight code flow, where individual request procs just look at a flag to decide whether to call the Xinerama or single screen version.

This isn't just much easier to understand (and debug), but also removes the need or the call vectors, thus allowing further simplification of the dispatcher.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net