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