Add experimental MPV player backend (#161)

Experimental MPV player backend

Related to #14, #22, & #85

You can switch to MPV in advanced settings and toggle using hardware
decoding or not.

This uses code and buildscripts from
https://github.com/mpv-android/mpv-android, plus other third party
libraries to build MPV

scripts/mpv/include/ci.sh

@@ -0,0 +1,91 @@
+#!/bin/bash -e
+
+# go to buildscripts root folder
+cd "$( dirname "${BASH_SOURCE[0]}" )/.."
+
+. ./include/depinfo.sh
+
+msg() {
+	printf '==> %s\n' "$1"
+}
+
+fetch_prefix() {
+	if [[ "$CACHE_MODE" == folder ]]; then
+		local text=
+		if [ -f "$CACHE_FOLDER/id.txt" ]; then
+			text=$(cat "$CACHE_FOLDER/id.txt")
+		else
+			echo "Cache seems to be empty"
+		fi
+		printf 'Expecting "%s",\nfound     "%s".\n' "$ci_tarball" "$text"
+		if [[ "$text" == "$ci_tarball" ]]; then
+			tar -xzf "$CACHE_FOLDER/data.tgz" -C prefix && return 0
+		fi
+	fi
+	return 1
+}
+
+build_prefix() {
+	msg "Building the prefix ($ci_tarball)..."
+
+	msg "Fetching deps"
+	IN_CI=1 ./include/download-deps.sh
+
+	# build everything mpv depends on (but not mpv itself)
+	for x in ${dep_mpv[@]}; do
+		msg "Building $x"
+		./buildall.sh $x
+	done
+
+	if [[ "$CACHE_MODE" == folder && -w "$CACHE_FOLDER" ]]; then
+		msg "Compressing the prefix"
+		tar -cvzf "$CACHE_FOLDER/data.tgz" -C prefix .
+		echo "$ci_tarball" >"$CACHE_FOLDER/id.txt"
+	fi
+}
+
+export WGET="wget --progress=bar:force"
+
+if [ "$1" = "export" ]; then
+	# export variable with unique cache identifier
+	echo "CACHE_IDENTIFIER=$ci_tarball"
+	exit 0
+elif [ "$1" = "install" ]; then
+	# install deps
+	if [[ -n "$ANDROID_HOME" && -d "$ANDROID_HOME" ]]; then
+		msg "Linking existing SDK"
+		mkdir -p sdk
+		ln -sv "$ANDROID_HOME" sdk/android-sdk-linux
+	fi
+
+	msg "Fetching SDK + NDK"
+	IN_CI=1 ./include/download-sdk.sh
+
+	msg "Fetching mpv"
+	mkdir -p deps/mpv
+	$WGET https://github.com/mpv-player/mpv/archive/master.tar.gz -O master.tgz
+	tar -xzf master.tgz -C deps/mpv --strip-components=1
+	rm master.tgz
+
+	msg "Trying to fetch existing prefix"
+	mkdir -p prefix
+	fetch_prefix || build_prefix
+	exit 0
+elif [ "$1" = "build" ]; then
+	# run build
+	:
+else
+	exit 1
+fi
+
+msg "Building mpv"
+./buildall.sh -n mpv || {
+	# show logfile if configure failed
+	[ ! -f deps/mpv/_build/config.h ] && cat deps/mpv/_build/meson-logs/meson-log.txt
+	exit 1
+}
+
+msg "Building mpv-android"
+./buildall.sh -n
+
+exit 0