[gstreamer] video playback broken with gstreamer older than 1.20

the gstreamer backend depends on the videoconvertscale element, which is only available in gstreamer 1.20 (released early 2022). t

• video playback on older systems will not work (ubuntu 22.04 comes with 1.20 already)

workarounds:

• users can update to more recent gstreamer
• the patch for qt is trivial