Adding a bit of configurability to the FXAA business. This alters the F3 Lab so that the number keys correspond to FXAA presets (with 0 being lowest and fastest, default being 3). Also adds the "-fxaa_preset" commandline argument, which accepts a single int parameter. New Shaders needed to take full advantage of this, see forum thread (http://www.hard-light.net/forums/index.php?topic=76145.0) for details.
