Use minimal prebuilt FFmpeg for Windows

On the scrcpy-deps repo, I built FFmpeg 5.1.2 binaries for Windows with
only the features used by scrcpy.

For comparison, here are the sizes of the dll for FFmpeg 5.1.2:
 - before: 89M
 - after: 4.7M

It also allows to upgrade the old FFmpeg version (4.3.1) used for win32.

Refs <https://github.com/rom1v/scrcpy-deps>
Refs <https://github.com/Genymobile/scrcpy/issues/1753>

app/prebuilt-deps/prepare-ffmpeg.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+set -e
+DIR=$(dirname ${BASH_SOURCE[0]})
+cd "$DIR"
+. common
+mkdir -p "$PREBUILT_DATA_DIR"
+cd "$PREBUILT_DATA_DIR"
+
+VERSION=5.1.2-scrcpy
+DEP_DIR="ffmpeg-$VERSION"
+
+FILENAME="$DEP_DIR".7z
+SHA256SUM=93f32ffc29ddb3466d669f7078d3fd8030c3388bc8a18bcfeefb6428fc5ceef1
+
+if [[ -d "$DEP_DIR" ]]
+then
+    echo "$DEP_DIR" found
+    exit 0
+fi
+
+get_file "https://github.com/rom1v/scrcpy-deps/releases/download/$VERSION/$FILENAME" \
+    "$FILENAME" "$SHA256SUM"
+
+mkdir "$DEP_DIR"
+cd "$DEP_DIR"
+
+ZIP_PREFIX=ffmpeg
+7z x "../$FILENAME"
+mv "$ZIP_PREFIX"/* .
+rmdir "$ZIP_PREFIX"