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/lua.sh

@@ -0,0 +1,44 @@
+#!/bin/bash -e
+
+. ../../include/path.sh
+
+if [ "$1" == "build" ]; then
+	true
+elif [ "$1" == "clean" ]; then
+	make clean
+	exit 0
+else
+	exit 255
+fi
+
+# Building seperately from source tree is not supported, this means we are forced to always clean
+$0 clean
+
+mycflags=(
+	# ensures correct linking into libmpv.so
+	-fPIC
+	# bionic is missing decimal_point in localeconv [src/llex.c]
+	-Dgetlocaledecpoint\\\(\\\)=\\\(46\\\)
+	# force fallback as ftello/fseeko are not defined [src/liolib.c]
+	-Dlua_fseek
+)
+
+# LUA_T= and LUAC_T= to disable building lua & luac
+# -Dgetlocaledecpoint()=('.') fixes bionic missing decimal_point in localeconv
+make CC="$CC" AR="$AR rc" RANLIB="$RANLIB" \
+	MYCFLAGS="${mycflags[*]}" \
+	PLAT=linux LUA_T= LUAC_T= -j$cores
+
+# TO_BIN=/dev/null disables installing lua & luac
+make INSTALL=${INSTALL:-install} INSTALL_TOP="$prefix_dir" TO_BIN=/dev/null install
+
+# make pc only generates a partial pkg-config file because ????
+mkdir -p $prefix_dir/lib/pkgconfig
+make pc >$prefix_dir/lib/pkgconfig/lua.pc
+cat >>$prefix_dir/lib/pkgconfig/lua.pc <<'EOF'
+Name: Lua
+Description:
+Version: ${version}
+Libs: -L${libdir} -llua
+Cflags: -I${includedir}
+EOF