contrib: qt: update to 6.8.0 LTS from 6.7.1
Qt 6.8 was released today: https://www.qt.io/blog/qt-6.8-released, it is a LTS release.
Next Qt minor release (6.9) is reportedly planned to be released in March 2025.
Minimal changes needed so far to continue supporting Windows 7 and 8.
Tested on Windows 7 and Windows 10.
I also fix the crash on exit with Windows7 video integration. See !6153 (merged).
I tried to order patches properly for qtbase (simply renamed, the actual apply ordering is already fine), the patches can be applied independently except the patch that disables linking d3d9, that requires Windows 7 compatibility patch to be applied before and the Windows 7 compatibility patch that requires the mutex reversion patch. I also added description to the patches I added as requested before.
I would like to update Qt patch releases as well, and this is possible even without updating the Qt tools image (currently, instead of bootstrapping Qt at each build, Qt tools are pre-supplied). Tools are meant to be compatible with patch releases, except qmlcachegen, which is not really crucial because Qt caches the QML files anyway at first encounter. I proposed to not provide qmlcachegen in !5737, so that I can update patch releases freely. Qt does not provide minor releases often, and patch releases fix important things which are relevant also for development. It was argued that qmlcachegen makes it possible to detect QML errors, yes, but that is really irrelevant because that tools is used for Debian build anyway.