Recognition segmentation image: add API function to set the background color

[stefaniapedrazzi]

Add a new Camera API function to set the background color of the segmented image. Something like: wb_camera_recognition_set_segmentation_background_color(WbDeviceTag tag, int color)

The color would have the same format for example as in the Display API: https://www.cyberbotics.com/doc/reference/display#wb_display_set_color

Default color will be black.