opencv3: Build with video input under darwin platforms

Adds the right dependencies for encoding video, and removes the cmake
flag preventing the feature to be compiled in.

pkgs/development/libraries/opencv/3.x.nix

···
 , enableDC1394    ? false, libdc1394
 , enableDocs      ? false, doxygen, graphviz-nox
 
-, AVFoundation, Cocoa, VideoDecodeAcceleration, bzip2
+, AVFoundation, Cocoa, VideoDecodeAcceleration, bzip2, CoreMedia, MediaToolbox
 }:
 
 assert blas.implementation == "openblas" && lapack.implementation == "openblas";
···
     # tesseract & leptonica.
     ++ lib.optionals enableTesseract [ tesseract leptonica ]
     ++ lib.optional enableTbb tbb
-    ++ lib.optionals stdenv.isDarwin [ bzip2 AVFoundation Cocoa VideoDecodeAcceleration ]
+    ++ lib.optionals stdenv.isDarwin [ bzip2 AVFoundation Cocoa VideoDecodeAcceleration CoreMedia MediaToolbox ]
     ++ lib.optionals enableDocs [ doxygen graphviz-nox ];
 
   propagatedBuildInputs = lib.optional enablePython pythonPackages.numpy
···
   ] ++ lib.optionals stdenv.isDarwin [
     "-DWITH_OPENCL=OFF"
     "-DWITH_LAPACK=OFF"
-    "-DBUILD_opencv_videoio=OFF"
   ] ++ lib.optionals enablePython [
     "-DOPENCV_SKIP_PYTHON_LOADER=ON"
   ] ++ lib.optionals enableEigen [

pkgs/top-level/all-packages.nix

···
   };
 
   opencv3 = callPackage ../development/libraries/opencv/3.x.nix {
-    inherit (darwin.apple_sdk.frameworks) AVFoundation Cocoa VideoDecodeAcceleration;
+    inherit (darwin.apple_sdk.frameworks) AVFoundation Cocoa VideoDecodeAcceleration CoreMedia MediaToolbox;
     ffmpeg = ffmpeg_4;
   };