Add Buffer Scalling Filter option

 Not sure if we want another option in the graphic settings:|. This add's an option to change filter used while scalling buffers

 Current behaviour is always linear, which looks very blurry if you stretch low res like x1 to high res window/fullscreen. This can decrease quality of some shader effects and is also probably hated by many "sharpness" fanatics out there that has to(~ upscalling glitches / weak hardware) use low res.

 Defaults to linear, so no change for those that will not use it.
It'd be a lot easier to read as SCALE_LINEAR and SCALE_NEAREST, in my opinion.