Add "Half skip" Mode

[sum2012]

My random idea from #16900 (comment)

Added as another mode to the speedhack setting Skip GPU readbacks.

May help #18607

[LunaMoo]

Why making it random if you want "half"(which would imply every other frame)?

Random pretty much means it might break things sometimes and sometimes it will not, doesn't feel like an useful thing to have, even less for performance reasons as lower framerate is better than unpredictable one.

[hrydgard]

One argument for making it random is that it will work to halve the work "evenly" whether the game does 1, 2 or 3 readbacks per frame for example - if you do every second readback, you run the risk of falling into stuck patterns and always skipping the same read every frame. But, obvious drawbacks are of course uneven framerates etc.

Either way, this idea of skipping readbacks randomly highly depends on what the game is doing, if it makes sense or not... I dunno if I want it in though, if we do add it, it'll be in speedhacks, which this PR does.

What game do you feel this is the most beneficial in, sum2012?

[sum2012]

MotorStorm Artic Edge, gpu readback the sun to adjustment the sky 's luminosity

[hrydgard]

Oh, yeah, I guess it can be a pretty good option for that game, since the effect isn't very frame-critical..

Hm. I'm gonna play with it soon.