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/scripts/mpv-android.sh

@@ -0,0 +1,77 @@
+#!/bin/bash -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+BUILD="$DIR/.."
+MPV_ANDROID="$DIR/../.."
+
+. $BUILD/include/path.sh
+. $BUILD/include/depinfo.sh
+
+if [ "$1" == "build" ]; then
+	true
+elif [ "$1" == "clean" ]; then
+	rm -rf $MPV_ANDROID/{app,.}/build $MPV_ANDROID/app/src/main/{libs,obj}
+	exit 0
+else
+	exit 255
+fi
+
+[ -n "$ANDROID_SIGNING_KEY" ] && BUNDLE=1
+
+nativeprefix () {
+	if [ -f $BUILD/prefix/$1/lib/libmpv.so ]; then
+		echo $BUILD/prefix/$1
+	else
+		echo >&2 "Warning: libmpv.so not found in native prefix for $1, support will be omitted"
+	fi
+}
+
+prefix32=$(nativeprefix "armv7l")
+prefix64=$(nativeprefix "arm64")
+prefix_x64=$(nativeprefix "x86_64")
+prefix_x86=$(nativeprefix "x86")
+
+if [[ -z "$prefix32" && -z "$prefix64" && -z "$prefix_x64" && -z "$prefix_x86" ]]; then
+	echo >&2 "Error: no mpv library detected."
+	exit 255
+fi
+
+PREFIX32=$prefix32 PREFIX64=$prefix64 PREFIX_X64=$prefix_x64 PREFIX_X86=$prefix_x86 \
+ndk-build -C app/src/main -j$cores
+
+targets=(assembleDebug)
+if [ -z "$DONT_BUILD_RELEASE" ]; then
+	targets+=(assembleRelease)
+	[ -n "$BUNDLE" ] && targets+=(bundleRelease)
+fi
+./gradlew "${targets[@]}"
+
+if [ -n "$ANDROID_SIGNING_KEY" ]; then
+	cd "${MPV_ANDROID}/app/build/outputs/apk"
+	apksigner=${ANDROID_HOME}/build-tools/${v_sdk_build_tools}/apksigner
+	for v in default api29; do
+		pushd $v
+		# sign the universal debug APK
+		"$apksigner" sign --ks "${ANDROID_SIGNING_KEY}" \
+			--in debug/app-$v-universal-debug.apk --out debug/app-$v-universal-debug-signed.apk
+		# but all of the release APKs
+		for apk in release/*-unsigned.apk; do
+			"$apksigner" sign --ks "${ANDROID_SIGNING_KEY}" \
+				--in $apk --out ${apk/-unsigned/-signed}
+		done
+		popd
+	done
+	# and the bundle
+	cd ../bundle
+	if [ -n "$BUNDLE" ]; then
+		if [ -z "$ANDROID_SIGNING_ALIAS" ]; then
+			echo >&2 "Error: ANDROID_SIGNING_ALIAS must be set to use jarsigner"
+			exit 1
+		fi
+		pushd defaultRelease
+		jarsigner -keystore "${ANDROID_SIGNING_KEY}" -signedjar \
+			app-default-release-signed.aab app-default-release.aab \
+			"${ANDROID_SIGNING_ALIAS}"
+		popd
+	fi
+fi