[MacPorts] #54064: port:mlt{, -qt5} : backported upstream patch, OpenCV & SDL tweaks

[MacPorts]

#54064: port:mlt{,-qt5} : backported upstream patch, OpenCV & SDL tweaks
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 Reporter:  RJVB         |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:  haspatch     |       Port:  mlt mlt-qt5
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 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).

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Ticket URL: <https://trac.macports.org/ticket/54064>
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