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/include/path.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && cd .. && pwd )"
+
+. "$DIR/include/depinfo.sh"
+
+os=linux
+[[ "$OSTYPE" == "darwin"* ]] && os=mac
+export os
+
+if [ "$os" == "mac" ]; then
+	[ -z "$cores" ] && cores=$(sysctl -n hw.ncpu)
+	# various things rely on GNU behaviour
+	export INSTALL=`which ginstall`
+	export SED=gsed
+else
+	[ -z "$cores" ] && cores=$(grep -c ^processor /proc/cpuinfo)
+fi
+cores=${cores:-4}
+
+# configure pkg-config paths if inside buildscripts
+if [ -n "$ndk_triple" ]; then
+	export PKG_CONFIG_SYSROOT_DIR="$prefix_dir"
+	export PKG_CONFIG_LIBDIR="$PKG_CONFIG_SYSROOT_DIR/lib/pkgconfig"
+	unset PKG_CONFIG_PATH
+fi
+
+toolchain=$(echo "$DIR/sdk/android-ndk-${v_ndk}/toolchains/llvm/prebuilt/"*)
+[ -d "$toolchain" ] && \
+	export PATH="$toolchain/bin:$DIR/sdk/android-ndk-${v_ndk}:$DIR/sdk/bin:$PATH"
+export ANDROID_HOME="$DIR/sdk/android-sdk-$os"
+unset ANDROID_SDK_ROOT ANDROID_NDK_ROOT