Allows camera to publish images in RGB24 format

AVT/Prosilica cameras publish images in Bayer8 by default, but this doesn't work for me. Fortunately, it's easy to put them in RGB24 format if the cameras support it. This code check for the pixel_format parameter and sets the camera pixel format appropriately (Bayer8 or Rgb24 only). Works with Mako and Manta cameras.