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/depinfo.sh

@@ -0,0 +1,43 @@
+#!/bin/bash -e
+
+## Dependency versions
+# Make sure to keep v_ndk and v_ndk_n in sync, both are listed on the NDK download page
+
+v_sdk=11076708_latest
+v_ndk=r29
+v_ndk_n=29.0.14206865
+v_sdk_platform=35
+v_sdk_build_tools=35.0.0
+
+v_lua=5.2.4
+v_unibreak=6.1
+v_harfbuzz=12.1.0
+v_fribidi=1.0.16
+v_freetype=2.14.1
+v_mbedtls=3.6.4
+
+
+## Dependency tree
+# I would've used a dict but putting arrays in a dict is not a thing
+
+dep_mbedtls=()
+dep_dav1d=()
+dep_ffmpeg=(mbedtls dav1d)
+dep_freetype2=()
+dep_fribidi=()
+dep_harfbuzz=()
+dep_unibreak=()
+dep_libass=(freetype2 fribidi harfbuzz unibreak)
+dep_lua=()
+dep_libplacebo=()
+dep_mpv=(ffmpeg libass lua libplacebo)
+dep_mpv_android=(mpv)
+
+
+## for CI workflow
+
+# pinned ffmpeg revision
+v_ci_ffmpeg=n8.0
+
+# filename used to uniquely identify a build prefix
+ci_tarball="prefix-ndk-${v_ndk}-lua-${v_lua}-unibreak-${v_unibreak}-harfbuzz-${v_harfbuzz}-fribidi-${v_fribidi}-freetype-${v_freetype}-mbedtls-${v_mbedtls}-ffmpeg-${v_ci_ffmpeg}.tgz"