This update for frei0r-plugins and libmlt fixes the following issues:

- frei0r-plugins:
+ Build against opencv-qt5-devel on Leap as well, Leap comes with the KF5/Qt5 based kdenlive (boo#930616)

- libmlt:
+ Build against Qt5 on Leap as well, the included kdenlive 15.08.1 (which is KF5/Qt5 based) will crash on startup otherwise (boo#930616)

- Restore pkgconfig(libavdevice), multiple providers should be solved with Prefer: pkg in prjconf

- Build with ffmpeg on Leap and Tumbleweed by default to make it a bit more useful (boo#954841)

- Replace pkgconfig(libavdevice) BuildRequires with libavdevice-devel to prevent an unresolveable choice between that and libavdevice-libav-devel

- Build against Qt5 on Leap as well, the included kdenlive 15.08.1 (which is KF5/Qt5 based) will crash on startup otherwise (boo#930616)