Since otherwise the include might not be visible and we need those now.

KImageFormats v6.0.0 is now available (which should be Qt 6-compatible)

This is still a bit flaky around when to remove a mention, but it should
work in most cases. Might add a toggle in the future to disable these
though.

* Remove -Dqt6 from gstreamer itself, since qmlgl is only part of gst-plugins-good
* Add necessary flags to gst-plugins-good

fixes #1600

Currently disabled by default.