Add screenshot max resolution limit

[weihuoya]

high resolution screenshot got large jpg file and really slow down SavedataScreen, especially on pc.
x1 is a little fuzzy, x2 is fine.

[unknownbrackets]

Which backend does it slow down? Maybe it's slow mipmap generation.

I guess this is fine, but maybe someone with 4K might consider 2x blurry...

-[Unknown]

[weihuoya]

test opengl and vulkan, GamePauseScreen slow down a little, too.

[LunaMoo]

Meanwhile most of the actual users actually don't care about screenshots freezing or doing whatever to the game as they use it to capture a moment and want to capture either eactly what they see as it works by default or even an extremely high res that could take a few seconds to make even if they loose post process filters. As such, I think this pr is unacceptable unless made optional and not enabled by default.

Edit: blah, didn't notice it's for savestate only, still I don't like forcing people to low res pictures for something that still captures a moment and some wish to view it later.

[hrydgard]

I'm fine with this as it's savestate only, it's still possible to take nice high res pictures if you want them, savestate screenshots are just for knowing where you are in the game. If anyone cares enough to have a hidden option to preserve full resolution, I would be OK with that too but really don't think this is worth a UI string.