Force images to be rgb8 internally, as expected

If the format is not explicitly specified, detection would not
work for the ROS images that are not rgb8
color_gui: Do not autoresize window, as it hides textboxes

There is probably a better fix (actually figuring out why
the gui elements disappear)
Merge branch 'hydro-devel' of https://github.com/uml-robotics/cmvision into hydro-devel