PlatformCore/Qt: Add sharp bilinear shader

[GranMinigun]

Does what it says on the tip. Helps with pixel irregularities at non-integer scaling factors, especially on lower output resolutions. Looks identical to nearest-neighbour interpolation at integer scaling factors and is not recommended to use with such configuration. Fairly cheap. Ported from RetroArch Slang shader collection. Tested in combination with all other shaders. Config and enum names are up for debate.

[fleroviux]

Thanks! This is working well on my end and the code is looking good to me. The only thing I am worried about is that it might be hard for users to tell the difference between nearest and sharp, especially when tested at integer scaling factors. Do you think it would make sense to deprecate the "nearest" option in favor of a "sharp" option which for efficiency falls back to the nearest code path at integer scales?

[GranMinigun]

I personally would prefer not to introduce any additional heuristics, to keep the code simpler. At this point, I think the shader is widespread and known enough to trust the user with a choice (see, e.g., ares, DOSBox, most recently Dolphin). And the cost of a mistake isn't that high — the shader runs just fine on mobile devices and SBCs like Raspberry Pi 4. Changing the default shader might be a better overall option.

[fleroviux]

Ah, alright. That's fair. I will go ahead and merge the PR then.