ofxBlackmagic should implement ofBaseVideoGrabber

[kylemcdonald]

it should be easier to jump between them, just like https://github.com/bakercp/ofxPS3EyeGrabber/blob/master/src/ofxPS3EyeGrabber.h#L21

the main difficulty is that currently the getPixels methods are all non-const so it's impossible to implement const versions.