clutter/frame-clock: Simplify next_presentation_time_us calculation

When more than one refresh interval has passed since last_presentation_time_us.

I honestly can't tell if the previous calculation was correct or not, but I'm confident the new one is, and it's simpler.

v2:

• ASCII art diagram didn't make sense anymore, try to improve (Ivan Molodetskikh)

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Split out from !3174 (merged), see !3174 (comment 1871910)