[GDK3] GdkFrameClockIdle: fix too-short update interval

Yariv requested to merge yarivb/gtk:gdk-3-fix-frame-clock-scheduling into gtk-3-24

The frame clock paint loop function was scheduled too often at certain circumstances - about twice per frame interval instead of only once. Fix that issue by scheduling the next invocation a full frame interval away instead of half an interval.

Fixes #2432 (closed)

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