stereo_image_proc: consistent image_transport

[mikeferguson]

• make image_transport work
• make remap work as expected with image_transport
• make subscribers lazy

[mikeferguson]

Testing notes:

• Can remap output properly (--ros-args -r points2:=test)
• Can remap individual camera topics:
  • -r left/image_raw:=test/image_raw remaps both left/image_raw and left/camera_info as expected
  • When also adding -p image_transport:=compressed we get the expected test/image_raw/compressed and test/camera_info
• Can remap camera_info topic - but it's slightly more complex, since you need to remap the remapped topic (I see no way to cleanly remap "left/camera_info" while allowing camera_info to automatically remap properly for when "left/image_raw" has been changed. This isn't a common use case, and is kind of outside the expected standard, so we will simply document it)
• Cannot remap just "left" and "right" - has to be "left/image_raw" - this is a bit different from ROS 1, but I think it is just how ROS 2 does lower level name spacing