Dev: Add missing GStreamer plugins for Gazebo video streaming

[litski0]

Added missing GStreamer plugins to installation instructions.

[Hwurzburg]

@rmackay9 ping

[khancyr]

doesn't match official instructions : https://github.com/ArduPilot/ardupilot_gazebo

Why do you think we need this ?

[litski0]

Well actually i had followed the docs step by step, but then the last example of gst-streamer which was to stream gazebo stream using gstreamer didnt worked
then i , ai 'ed the problem , turned out i was missing the packages
after running the depencies it worked
therefore i raised the pull reuqest

is there any other way i could verify the changes i had proposed, one way i have is to remove and reinstall but am currently in middle of a project would like to avoid it, dont want it to be a ai sloped pr

[rmackay9]

Hi @Hwurzburg, I'm not too familiar with this area so I'll leave it to @khancyr to make the call on whether it is correct or not

@litski0, thanks very much for noticing the problem and creating this PR!