present: Rename window_msc variable in present function

The value is not the current msc of the window, but the target value
the client sets independently of the window speicific msc offset. Make
this clearer.

Signed-off-by: Roman Gilg <subdiff@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
4 jobs for cleanupTargetMscCalc in 5 minutes and 7 seconds (queued for 5 seconds)