[WIP] Two video streams

[d0vgan]

[WIP] Two video streams

Description

You can treat this as a lite alternative to the #8017 .
To be honest, I don't really know QML, and this is just a small attempt to add the support of 2 video streams to QGC.
Here is what it does:

• it assumes there are 2 cameras, and each of these cameras has an HQ and a LQ videostream;
• an "Enable auxiliary video streams" checkbox has been added to QGC's Video Settings;
• once the "Enable auxiliary video streams" checkbox, you can specify the Cam 1 HQ, Cam 2 LQ, Cam 2 HQ and Cam 1 LQ streams there.

This is work-in-progress and demonstrates the current state. Any hints and contributions are welcome.

Test Steps

1. Start mediamtx: ./mediamtx
2. Start video stream 1: ffmpeg -re -stream_loop -1 -i anim1.mp4 -c copy -f rtsp rtsp://127.0.0.1:8554/stream1
3. Start video stream 2: ffmpeg -re -stream_loop -1 -i anim2.mp4 -c copy -f rtsp rtsp://127.0.0.1:8554/stream2
4. Start QGC and specify the given streams in the Video Settings
5. In QGC's window, right-click the small video and select "Switch to auxiliary video stream"

And here is the problem: currently, the switching between the video streams is very "heavy". This is how the video receivers seem to work. Maybe this can be improved by updating the GStreamer-related code, but I don't have such expertise.

Related Issue

#8017

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