mlt: update to 7.8.0

7.8.0
This highlight of this version is a new glaxnimate producer to render 2D vector
art and animation.

7.6.0
This version adds image slice-threading to many filters and full support for
full range color. All inputs are normalized to and processed at the range
specified by the consumer property color_range that defaults to tv/mpeg (limited).

7.4.0
This main highlight of this version is property animation for avfilter!

7.2.0
This is the first major maintenance release for the new major version 7
rendering it much more production ready. Plus there are a few nice new features.

(markd)