Improve crossbuild

Install all the prebuilt dependencies for Windows to a specific folder,
and use meson command line options to specify their location.

This removes crossbuild-specific code from the meson scripts and will
simplify dependency upgrades.

PR #4460 <https://github.com/Genymobile/scrcpy/pull/4460>

app/prebuilt-deps/prepare-ffmpeg.sh

@@ -6,11 +6,11 @@ cd "$DIR"
 mkdir -p "$PREBUILT_DATA_DIR"
 cd "$PREBUILT_DATA_DIR"
 
-VERSION=6.1-scrcpy-2
+VERSION=6.1-scrcpy-3
 DEP_DIR="ffmpeg-$VERSION"
 
 FILENAME="$DEP_DIR".7z
-SHA256SUM=7f25f638dc24a0f5d4af07a088b6a604cf33548900bbfd2f6ce0bae050b7664d
+SHA256SUM=b646d18a3d543a4e4c46881568213499f22e4454a464e1552f03f2ac9cc3a05a
 
 if [[ -d "$DEP_DIR" ]]
 then