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/libplacebo.sh

@@ -0,0 +1,25 @@
+#!/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 setup $build --cross-file "$prefix_dir"/crossfile.txt \
+	-Dvulkan=disabled -Ddemos=false
+
+ninja -C $build -j$cores
+DESTDIR="$prefix_dir" ninja -C $build install
+
+# add missing library for static linking
+# this isn't "-lstdc++" due to a meson bug: https://github.com/mesonbuild/meson/issues/11300
+${SED:-sed} '/^Libs:/ s|$| -lc++|' "$prefix_dir/lib/pkgconfig/libplacebo.pc" -i