Move prebuilt-deps/ to app/

The prebuilt dependencies are specific to the client app (not the
server).

This also avoids to reference the parent directory (../) from
app/meson.build.

app/prebuilt-deps/prepare-ffmpeg-win64.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+set -e
+DIR=$(dirname ${BASH_SOURCE[0]})
+cd "$DIR"
+. common
+mkdir -p "$PREBUILT_DATA_DIR"
+cd "$PREBUILT_DATA_DIR"
+
+DEP_DIR=ffmpeg-win64-5.0
+
+FILENAME=ffmpeg-5.0-full_build-shared.7z
+SHA256SUM=e5900f6cecd4c438d398bd2fc308736c10b857cd8dd61c11bcfb05bff5d1211a
+
+if [[ -d "$DEP_DIR" ]]
+then
+    echo "$DEP_DIR" found
+    exit 0
+fi
+
+get_file "https://github.com/GyanD/codexffmpeg/releases/download/5.0/$FILENAME" \
+    "$FILENAME" "$SHA256SUM"
+
+mkdir "$DEP_DIR"
+cd "$DEP_DIR"
+
+ZIP_PREFIX=ffmpeg-5.0-full_build-shared
+7z x "../$FILENAME" \
+    "$ZIP_PREFIX"/bin/avutil-57.dll \
+    "$ZIP_PREFIX"/bin/avcodec-59.dll \
+    "$ZIP_PREFIX"/bin/avformat-59.dll \
+    "$ZIP_PREFIX"/bin/swresample-4.dll \
+    "$ZIP_PREFIX"/bin/swscale-6.dll \
+    "$ZIP_PREFIX"/include
+mv "$ZIP_PREFIX"/* .
+rmdir "$ZIP_PREFIX"