Basic video encoder changer (viWidth), affects GC/Wii only

Default value is 640, the user can change in the config the value they want; up to 720.
Currently (video options-wise) only works as a display, and only by manually changing the config it will work.

*Tested on Wii, with both 4:3 and 16:9 modes but needs testing on GameCube.

This can be used to correct PARs
For more info: http://www.libretro.com/forums/viewtopic.php?f=31&t=2223