Custom PPSSPP shaders

[LunaMoo]

I don't care about Xenia, but there's nothing Nvidia or AMD has that's done by post process which is worth porting to PPSSPP which is why nobody did it yet:
- full FXAA version is meh compared to simply using SSAA(the FXAA we have is much lighter version that's nice to use on lower end hardware like smartphones),
- AMD FSR is a picture-degrading performance feature which might be useful for complex modern games that have problems running at 4k on high end hardware, it however is pretty useless in PSP games that easily render at 8k and more even on mediocre gpu's,
- CAS is just sharpening, we have sharpen filters that does the same, the thing is sharpening is opposite to AA which is especially noticeable at low res and most people choose AA.


PPSSPP has it's own post process shader support which is way more robust than in other emulators, just to name a few unique features:
- ability to set different effects when video is playing,
- ability to affect post process by game memory changes(via special ppsspp-only cwcheat) some ideas how this can be used is to for example flash the screen on hit, make a blur effect above certain speed, reduce colors on hp drop, add bloom when spells are casted etc.,
- auto-translation from GLSL to all backends that support post process shaders(meaning for all except D3D9 which is kept as a legacy backend for really rare cases of awfully old windows based hardware),
- multi pass shaders where passes can be easily controlled via UI.