added static function to set preferred subtype for Directshow capture / Windows

[ofTheo]

See forum thread here:

https://forum.openframeworks.cc/t/issue-with-videograbber-performance-at-1080p-30fps-in-ofdirectshowgrabber/43948/7

usage:

#include "ofDirectShowGrabber.h"

ofDirectShowGrabber::setPreferredFormat((int)VI_MEDIASUBTYPE_MJPG);
//ofDirectShowGrabber::setPreferredFormat(-1); //clear the preferred type

//then your initGrabber code