(GX) Made "Set Screen Width" editable and added deflicker filtering as an option

Tested and working but both require a restart/load game to take effect, it's possible to update the screen but I don't know how.

Previously the deflicker filter was on by default for both 480i and 480p, disabling the filter provides a small but neat improvement to quality. Most if not all Virtual Console games don't use deflicker in 480 mode.