manage ffmpeg dependency w/ hunter

This adds support for ffmpeg as a hunter package w/ appropriate options: `-DWITH_FFMPEG=ON -DOPENCV_FFMPEG_USE_FIND_PACKAGE=ON`

* update Hunter release
* update HunterGate
* enable ffmpeg package w/ `if(HUNTER_ENABLED)`
* translate ffmpeg CONFIG libs to CMake's variable style usage,
  i.e. `${FFMPEG_INCLUDE_DIRS}, ${FFMPEG_LIBRARIES}`

Author: headupinclouds

cmake/OpenCVFindLibsVideo.cmake

@@ -186,6 +186,8 @@ endif(WITH_XIMEA)
 ocv_clear_vars(HAVE_FFMPEG)
 if(WITH_FFMPEG)  # try FFmpeg autodetection
   if(OPENCV_FFMPEG_USE_FIND_PACKAGE)
+    if(NOT HUNTER_ENABLED)
+
     if(OPENCV_FFMPEG_USE_FIND_PACKAGE STREQUAL "1" OR OPENCV_FFMPEG_USE_FIND_PACKAGE STREQUAL "ON")
       set(OPENCV_FFMPEG_USE_FIND_PACKAGE "FFMPEG")
     endif()
@@ -195,6 +197,23 @@ if(WITH_FFMPEG)  # try FFmpeg autodetection
     else()
       message(STATUS "Can't find FFmpeg via find_package(${OPENCV_FFMPEG_USE_FIND_PACKAGE})")
     endif()
+
+    else(HUNTER_ENABLED)
+      hunter_add_package(ffmpeg)
+      find_package(ffmpeg CONFIG REQUIRED)
+
+      set(FFMPEG_FOUND TRUE)
+      foreach(lib avcodec avformat avutil swresample swscale)
+        get_target_property(
+          ${lib}_INCLUDE_DIR
+          ffmpeg::${lib}
+          INTERFACE_INCLUDE_DIRECTORIES
+          )
+        list(APPEND FFMPEG_INCLUDE_DIRS "${${lib}_INCLUDE_DIR}")
+        list(APPEND FFMPEG_LIBRARIES "ffmpeg::${lib}")
+      endforeach()
+      list(REMOVE_DUPLICATES FFMPEG_INCLUDE_DIRS)
+    endif()
   elseif(WIN32 AND NOT ARM AND NOT OPENCV_FFMPEG_SKIP_DOWNLOAD)
     include("${OpenCV_SOURCE_DIR}/3rdparty/ffmpeg/ffmpeg.cmake")
     download_win_ffmpeg(FFMPEG_CMAKE_SCRIPT)