Apply the xBR high-quality magnification filter which is designed for pixel art. It follows a set of edge-detection rules, see http://www.libretro.com/forums/viewtopic.php?f=6&t=134.

It accepts the following option:

n

Set the scaling dimension: 2 for 2xBR, 3 for 3xBR and 4 for 4xBR. Default is 3.