Merge pull request #22706 from kallaballa:libavdevice_for_ffmpeg_v4l2

Introduce libavdevice to make v4l2 available to the ffmpeg backend

* introduce libavdevice to make v4l2 available to the ffmpeg backend

* downgrade the min required libavdevice version to 53.2.0

* make libavdevice optional

* create OCV_OPTION OPENCV_FFMPEG_ENABLE_LIBAVDEVICE and add definition through ocv_add_external_target

* move OCV_OPTION 'OPENCV_FFMPEG_ENABLE_LIBAVDEVICE' to detect_ffmpeg.cmake

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -95,6 +95,9 @@ extern "C" {
 
 #include <libavcodec/avcodec.h>
 #include <libswscale/swscale.h>
+#ifdef HAVE_FFMPEG_LIBAVDEVICE
+#include <libavdevice/avdevice.h>
+#endif
 
 // https://github.com/FFmpeg/FFmpeg/blob/b6af56c034759b81985f8ea094e41cbd5f7fecfb/doc/APIchanges#L602-L605
 #if LIBAVFORMAT_BUILD < CALC_FFMPEG_VERSION(58, 9, 100)
@@ -625,6 +628,10 @@ struct CvCapture_FFMPEG
 
 void CvCapture_FFMPEG::init()
 {
+#ifdef HAVE_FFMPEG_LIBAVDEVICE
+    //libavdevice is available, so let's register all input and output devices (e.g v4l2)
+    avdevice_register_all();
+#endif
     ic = 0;
     video_stream = -1;
     video_st = 0;