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)
Status Job ID Name Coverage
  Docker Image
passed #1658171
debian-testing

00:00:21

 
  Build And Test
passed #1658172
autotools

00:04:46

passed #1658173
meson

00:04:36

passed #1658174
meson-noglamor

00:01:08