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/libass.sh

@@ -0,0 +1,25 @@
+#!/bin/bash -e
+
+. ../../include/path.sh
+
+if [ "$1" == "build" ]; then
+	true
+elif [ "$1" == "clean" ]; then
+	rm -rf _build$ndk_suffix
+	exit 0
+else
+	exit 255
+fi
+
+[ -f configure ] || ./autogen.sh
+
+mkdir -p _build$ndk_suffix
+cd _build$ndk_suffix
+
+../configure \
+	--host=$ndk_triple --with-pic \
+	--enable-static --disable-shared \
+	--enable-libunibreak --disable-require-system-font-provider
+
+make -j$cores
+make DESTDIR="$prefix_dir" install