OpenCV 4 build issue

[tkircher]

Between OpenCV 3 and 4, CV_LOAD_IMAGE_UNCHANGED changed to IMREAD_UNCHANGED. So to get this module to build with OpenCV 4, make this change in image_transport_plugins/compressed_depth_image_transport/src/codec.cpp, line 134:

cv_ptr->image = cv::imdecode(imageData, IMREAD_UNCHANGED);

Noetic is supposed to support OpenCV 4, but there are very few changes that need to be made for Melodic to support it. Few people use OpenCV 3 anymore.