Revision 563430

graphics/opencv: update to 4.5.1 -- and make port more easily maintainable

This is a major upgrade from 3.x to 4.x.

Changelog from versions 3.4.1--4.5.1 can be found here:
    https://github.com/opencv/opencv/wiki/ChangeLog

Note: this has explicitely not been added as a new graphics/opencv4 port, but replaces the
      previous graphics/opencv[3] port. Again, to improve maintainability by not giving ports
      the option to pick the "wrong one" - this leads however to some abandoned ports being
      broken.

The port has been greatly simplified:
    * graphics/opencv-core which existed to enable ffmpeg to depend on opencv, and vice versa
      has been removed. ffmpeg no longer can depend on opencv.
    * graphics/py-opencv has been integrated into graphics/opencv, the default versions python
      bindings will be built unless the PYTHON option is explicitely turned off.
    * graphics/opencv-java has been integrated into graphics/opencv -- it is off by default,
      but can be enabled by toggling the JAVA option -- there are no consumers in the tree,
      so that option might go away in the future.
    * All the previous options have been removed and replaced by a (hopefully) sane set of
      dependencies that make the port and package most usable for the majority of consumers.
      - Please let me know if you think there are better defaults (i.e. anything that is missing,
        or something that should not be dependet on).
      - If you think something should be added or removed, please open a bug report.
      - If you think something should be added as an optional dependency, please open a
        bug report (with a good reason [tm]).

The depending ports have been updated to work against opencv4, or marked broken.
    * Ports broken:
      - graphics/rubygem-objectdetect: OpenCV4 no longer ships opencv-1.0 API
      - graphics/p5-Image-ObjectDetect: OpenCV4 no longer ships opencv-1.0 API
      - graphics/gimp-gmic-plugin: OpenCV4 no longer ships opencv-1.0 API
    * Backports:
      - misc/visp: https://github.com/lagadic/visp/commit/dfa7e4bd47c24cd2e631477e6afb18806733ff8a
      - multimedia/zart: https://github.com/c-koi/zart/commit/6ca1964690afbbc78627d7c868a692401043a584,
                         https://github.com/c-koi/zart/commit/d3a2931b1a07ec0322211f253468000363c4b6cb
    * Others:
      - misc/actiona: switch to pkgconfig 'opencv4'
      - multimedia/libav: drop opencv support
      - misc/darknet: already failed to build prior to the upgrade
      - math/saga: remove patching added to work against opencv3