CleanCode: placement: pass rects to areasOnSameMonitor() as values

Enrico Weigelt requested to merge metux/xfwm4:submit/areaOnSameMonitor into master

The structs are very small and just input parameters (the function should never change them), so can be easily passed as values instead of pointers. Making the code a bit easier to understand (the prototype alone makes clear that these are pure inputs), and - depending on cpu arch - gives compiler chance for better optimizations (register passing, l0 cache utilization, etc).

Signed-off-by: Enrico Weigelt, metux IT consult

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