ci/windows: Copy downloaded FFmpeg libraries

Copies FFmpeg libraries that were downloaded during the CMake
configuration. Fixes dynamic linking issues with the MinGW builds.

.ci/scripts/windows/docker.sh

@@ -42,3 +42,8 @@ done
 pip3 install pefile
 python3 .ci/scripts/windows/scan_dll.py package/*.exe "package/"
 python3 .ci/scripts/windows/scan_dll.py package/imageformats/*.dll "package/"
+
+# copy FFmpeg libraries
+EXTERNALS_PATH="$(pwd)/build/externals"
+FFMPEG_DLL_PATH="$(find ${EXTERNALS_PATH} -maxdepth 1 -type d | grep ffmpeg)/bin"
+find ${FFMPEG_DLL_PATH} -type f -regex ".*\.dll" -exec cp -v {} package/ ';'