video_output: shuffle some code around to make it clearer who does what

• move the VOUT_MWAIT_TOLERANCE use closer to the wait calls (independent from !445 (merged))
• DisplayPicture() only returns actual picture dates where it expects to be called, the vout loop takes care of adjusting the wait it will do to call DisplayPicture() at that date
• explicitly handle the case when displayed.current is set but was not displayed, so we don't have initialized variables that don't have a general use case.
• remove superflous variables when other ways are possible (dropped_current_frame)
• only use the refresh bool to tell if we need to do the rendering or not

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v7:

• rebased
• adjusted the code to the current master (so commits are gone)
• added more explanations in some commits