YUV pixel formats, support for pixelFormats in ofPixels and YUV in ofGstVideoGrabber

this adds some new pixel formats related to YUV + support for them in ofPixels. some functionality is not implemented yet, like setting or getting a color from a yuv ofPixels, cropping them... but it does the allocation correctly

it also adds yuv support for the gstreamer videograbber although the textures only show the Y plane as luminance setting the texture to false in the videograbber and using shaders for the color conversion will help with performance in platforms like the rpi or mobile
Spelling on `OF_PIXELS_UNKOWN` -> `OF_PIXELS_UNKNOWN` throughout.