[Vita][Feature Request] Deprecate Piglet builds in favour of PVR_PSP2, bringing GLSL shaders support

[swosho]

While Pigs In A Blanket works, CG shaders have to be manually adapted to even make them useable (#12414). In comparison, PVR_PSP2 natively supports GLSL shaders, and it's potentially a better performing solution as well.