VideoCommon: rewrite frame skipping code

[degasus]

No description provided.

[shuffle2]

lgtm

[skidau]

Interesting. In which cases can we see this code in action? Are movies that write directly to the XFB frameskipped?

[JMC47]

Frameskipping has not worked for me for a very long time; whenever I turn on frameskip, I lose all rendering whenever Dolphin slows down until I hit 60 again, which means it's not very useful for me. Unable to test, as such.

[degasus]

@skidau oh, I have only touched the way howto skip a frame, but I haven't touched the code which handles the logic which frame should be skipped. But it did work for me, so I thought it is already fine. Do you want me to also change this part or is this PR also fine without any UI changes?

[skidau]

@degasus, ok looks good to me

[skidau]

@degasus: This comment grants you the permission to merge this pull request whenever you think it is ready. After addressing the remaining comments, click this link to merge.

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@dolphin-emu-bot allowmerge

[TurboK234]

Frame skipping has worked for me in some situations, as I'm heavily GPU restricted with Intel HD 4600 and i5 4670S, dropping fps to 30 while keeping the game speed normal whenever unable to maintain 60. Especially SMG1/SMG2 seemed to work ok this way.
But still I got some longer "freezes" when the frameskipping logic seemed to lag too much, which ended in sudden deaths ect, so I'm not using it anymore. Especially when Zelda ucode HLE fix made it possible to play without any FPS dips, basically.
This isn't in scope of this PR, but @skidau was wondering about frameskipping in general...