video_streaming - Update QGC streaming doc link

[hamishwillee]

Fixes #3430

[hamishwillee]

@DonLakeFlyer @HTRamsey This is the source for the PX4 instructions for installing the GStreamer on Ubuntu for working with QGC.

The linked https://github.com/mavlink/qgroundcontrol/blob/master/src/VideoManager/VideoReceiver/GStreamer/README.md suggests something like this

Suggested change

	sudo apt install gstreamer1.0-plugins-bad gstreamer1.0-libav gstreamer1.0-gl -y
	list=$(apt-cache --names-only search ^gstreamer1.0-* | awk '{ print $1 }' | sed -e /-doc/d | grep -v gstreamer1.0-hybris)
	sudo apt-get install $list
	sudo apt-get install libgstreamer-plugins-base1.0-dev libgstreamer-plugins-bad1.0-dev -y

Is the PX4 variant "wrong" - i.e. for Ubuntu can I leave it as it is, or should we be using the same instructions as in QGC tree?

[HTRamsey]

The PX4 variant just includes runtime dependencies (which are included in the AppImage in v5, unlike v4.X) while the QGC one includes build-time libs but also unnecessarily installs literally everything gstreamer related. We've just been relying on a build script (https://github.com/mavlink/qgroundcontrol/blob/master/tools/setup/install-dependencies-debian.sh) that we can update without having to update instructions like these in the future.

There is also a good likelihood that the main/default video receiver switches to QGC provided ffmpeg libs which would mean no additional libs are required and would resolve all the problems that have existed due to GStreamer's difficulty to work with.

Not 100% sure how to handle this at the moment.

[hamishwillee]

Thanks very much @HTRamsey !

OK, so I have added a note that this step is not required for daily builds and releases after QGC4 - is that correct? (or is it only correct for Appimages)

1. So what of step 1 in https://docs.qgroundcontrol.com/master/en/qgc-user-guide/getting_started/download_and_install.html#ubuntu is no longer required for daily builds? As this is "main" we should document the daily build behaviour - but with a note that for release builds you still have to do the old steps.
2. Does it make sense to update https://github.com/mavlink/qgroundcontrol/blob/master/src/VideoManager/VideoReceiver/GStreamer/README.md to refer to https://github.com/mavlink/qgroundcontrol/blob/master/tools/setup/install-dependencies-debian.sh as well?

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