The prebuilt dependencies are specific to the client app (not the
server).
This also avoids to reference the parent directory (../) from
app/meson.build.
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.
Build the Windows binary from mingw on Linux, using the official
prebuilt binaries for ffmpeg, SDL2 and adb.
MSYS2 and all its packaged dll are not necessary anymore.
