Add Buffer Scalling Filter option

 Not sure if we want another option in the graphic settings:|, so feel free to close it if it's not usefull. It 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 also decrease quality of some shader effects and is also probably hated by many "sharpness" fanatics and 2D purists out there, who often think non-buffered rendering is best simply because it's 1:1 and free of blur.