Add full support for libretro shaders

[FunkyFr3sh]

Load .glslp settings file instead of .glsl shader directly to support all shaders including multi-pass ones.

[Artoria2e5]

Some strange-ratioed shaders might also benefit from this, 4xbrz and 6xbrz for example. At least I didn't read anything about the scale ratios in the glsl files and only found them in glslp.

[Disaer]

Agreed, much want to try NNEDI3 on Project Diablo 2.

[maciejkpl]

Is there any chance for this?