port:mlt{,-qt5} : backported upstream patch, OpenCV & SDL tweaks

[RJVB (René Bertin)]

This patch introduces the following 4 enhancements (besides cleanup to follow Qt5 changes):

• backports a fix from the next release that prevents crashing on exit in dependents using the C++ MLT framework (Kdenlive)
• introduces a gpl3 variant to enable some more features
• introduces an opencv variant to avoid opportunistic dependencies and to formalise the fact that the choice of Qt version should match in OpenCV and Melt .
• introduces a disable_sdl variant which can be useful when when Melt is used in dependents that themselves depend on SDL2, like Kdenlive. This will help avoid (warnings about) runtime conflicts which could lead to unforeseen issues. It can be hoped that one day MLT will support SDL2 too. I have not noticed any feature loss (in Kdenlive).

[RJVB (René Bertin)]

backported upstream fix

[RJVB (René Bertin)]

On a related note: adding Movit support for hardware video processing would be nice! ​https://movit.sesse.net/

[mf2k (Frank Schima)]

in the mlt patch, please use the normal syntax for adding a revision line and not append it to the version line. Also, revision should be 1, not 2, since it starts at 0.

[RJVB (René Bertin)]

I think Dan can handle those points if he agrees.

Me I think that adding the revision statement on the same line as the version is the best and simplest protection against forgetting to remove it; it also reads more in line with how the port effective version is shown elsewhere. Both alternatives are valid syntax so I don't see why we have to be splitting hairs over this.

[ddennedy (Dan Dennedy)]

There is a lot of changes in here. Will you volunteer to be the new maintainer of the mlt port?

[RJVB (René Bertin)]

Co/principal maintainer, sure, esp. of the Qt5 side of things. I don't have commit rights so I'm not the most ideal candidate to take over maintainership completely.

[ddennedy (Dan Dennedy)]

I do not have commit access either, but I am ok with co-maintainer as well.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

I believe all of these proposals have been incorporated.
If not, please feel free to reopen.