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/get_dependencies.sh

@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+
+set -exou pipefail
+
+BUILD_PATH="./deps"
+
+mkdir -p "$BUILD_PATH"
+pushd "$BUILD_PATH" || exit
+
+function clone(){
+  repo=$1
+  branch=$2
+  dir=$3
+  shift 3
+
+  if [[ -d "$dir" ]]; then
+    pushd "$dir" || exit
+    git checkout --force "$branch"
+    popd || exit
+  else
+    git clone "$repo" --depth 1 --single-branch -b "$branch" "$dir" "$@"
+  fi
+}
+
+clone "https://github.com/videolan/dav1d" "1.5.2" dav1d
+
+clone "https://github.com/FFmpeg/FFmpeg" "n8.0" ffmpeg
+
+clone "https://gitlab.freedesktop.org/freetype/freetype.git" "VER-2-14-1" freetype2 --recurse-submodules
+
+clone "https://github.com/libass/libass" "master" libass
+
+clone "https://github.com/haasn/libplacebo" "master" libplacebo --recurse-submodules
+
+clone "https://github.com/mpv-player/mpv" "master" mpv
+
+if [[ ! -d mbedtls ]]; then
+	mkdir mbedtls
+	wget https://github.com/Mbed-TLS/mbedtls/releases/download/mbedtls-3.6.4/mbedtls-3.6.4.tar.bz2 -O - | \
+		tar -xj -C mbedtls --strip-components=1
+fi
+
+if [[ ! -d fribidi ]]; then
+	mkdir fribidi
+	wget https://github.com/fribidi/fribidi/releases/download/v1.0.16/fribidi-1.0.16.tar.xz -O - | \
+		tar -xJ -C fribidi --strip-components=1
+fi
+
+if [[ ! -d harfbuzz ]]; then
+	mkdir harfbuzz
+	wget https://github.com/harfbuzz/harfbuzz/releases/download/12.1.0/harfbuzz-12.1.0.tar.xz -O - | \
+		tar -xJ -C harfbuzz --strip-components=1
+fi
+
+version_unibreak="6.1"
+if [[ ! -d unibreak ]]; then
+	mkdir unibreak
+	wget https://github.com/adah1972/libunibreak/releases/download/libunibreak_${version_unibreak//./_}/libunibreak-${version_unibreak}.tar.gz -O - | \
+		tar -xz -C unibreak --strip-components=1
+fi
+
+if [[ ! -d lua ]]; then
+	mkdir lua
+	wget https://www.lua.org/ftp/lua-5.2.4.tar.gz -O - | \
+		tar -xz -C lua --strip-components=1
+fi
+
+# python packages: jsonschema jinja2 meson
+
+popd || exit