Do not try to build sox without sox.pc or libst-config (SF-196).
Check for libswscale and libavdevice when configuring (SF-196).
Improve installation documentation (SF-196).
Fix segfault on consumer-fatal-error event.
Fix muxing vorbis into webm/mkv with libavcodec < 55.
Add producer xml-nogl to disable auto-qglsl creation.

Sometimes you want to load MLT XML while ignoring the presence of OpenGL
filters and transitions.
Fix compile warnings on string literal for default parameter.