Improve prebuilt system

This aims to fix two issues with the previous implementation:
 1. the whole content of downloaded archives were extracted, while only
    few files are necessary;
 2. the archives were extracted in the prebuild-deps/ directory as is.

As a consequence of (2), the actual directory name relied on the root
directory of the archive. For adb, this root directory was always
"platform-tools", so when bumping the adb version, the target directory
already existed and the dependency was not upgraded (the old one had to
be removed manually).

Expose common function to download a file and check its checksum, but
let the custom script for each dependency extract only the needed files
and reorganize the content if necessary.

prebuilt-deps/prepare-ffmpeg-win32.sh

@@ -0,0 +1,45 @@
+#!/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-win32-4.3.1
+
+FILENAME_SHARED=ffmpeg-4.3.1-win32-shared.zip
+SHA256SUM_SHARED=357af9901a456f4dcbacd107e83a934d344c9cb07ddad8aaf80612eeab7d26d2
+
+FILENAME_DEV=ffmpeg-4.3.1-win32-dev.zip
+SHA256SUM_DEV=230efb08e9bcf225bd474da29676c70e591fc94d8790a740ca801408fddcb78b
+
+if [[ -d "$DEP_DIR" ]]
+then
+    echo "$DEP_DIR" found
+    exit 0
+fi
+
+get_file "https://github.com/Genymobile/scrcpy/releases/download/v1.16/$FILENAME_SHARED" \
+    "$FILENAME_SHARED" "$SHA256SUM_SHARED"
+get_file "https://github.com/Genymobile/scrcpy/releases/download/v1.16/$FILENAME_DEV" \
+    "$FILENAME_DEV" "$SHA256SUM_DEV"
+
+mkdir "$DEP_DIR"
+cd "$DEP_DIR"
+
+ZIP_PREFIX_SHARED=ffmpeg-4.3.1-win32-shared
+unzip "../$FILENAME_SHARED" \
+    "$ZIP_PREFIX_SHARED"/bin/avutil-56.dll \
+    "$ZIP_PREFIX_SHARED"/bin/avcodec-58.dll \
+    "$ZIP_PREFIX_SHARED"/bin/avformat-58.dll \
+    "$ZIP_PREFIX_SHARED"/bin/swresample-3.dll \
+    "$ZIP_PREFIX_SHARED"/bin/swscale-5.dll
+
+ZIP_PREFIX_DEV=ffmpeg-4.3.1-win32-dev
+unzip "../$FILENAME_DEV" \
+    "$ZIP_PREFIX_DEV/include/*"
+
+mv "$ZIP_PREFIX_SHARED"/* .
+mv "$ZIP_PREFIX_DEV"/* .
+rmdir "$ZIP_PREFIX_SHARED" "$ZIP_PREFIX_DEV"