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/dav1d.sh

@@ -0,0 +1,22 @@
+#!/bin/bash -e
+
+. ../../include/path.sh
+
+build=_build$ndk_suffix
+
+if [ "$1" == "build" ]; then
+	true
+elif [ "$1" == "clean" ]; then
+	rm -rf $build
+	exit 0
+else
+	exit 255
+fi
+
+unset CC CXX # meson wants these unset
+
+meson setup $build --cross-file "$prefix_dir"/crossfile.txt \
+	-Denable_tests=false -Db_lto=true -Dstack_alignment=16
+
+ninja -C $build -j$cores
+DESTDIR="$prefix_dir" ninja -C $build install