Fixes MPV crash when playing next up (#772)

## Description
Ensure that MPV is always destroyed before being initialized again.
Also, now creating a new player is only done when switching backends
which means faster, more efficient playing the next up.

### Related issues
Fixes #766

app/src/main/java/com/github/damontecres/wholphin/ui/playback/PlaybackViewModel.kt

@@ -219,7 +219,9 @@ class PlaybackViewModel
                     PlayerBackend.PREFER_MPV -> if (isHdr) PlayerBackend.EXO_PLAYER else PlayerBackend.MPV
                 }
 
-            Timber.i("Selected backend: %s", playerBackend)
+            Timber.d("Selected backend: %s", playerBackend)
+            if (currentPlayer.value?.backend != playerBackend) {
+                Timber.i("Switching player backend to %s", playerBackend)
                 withContext(Dispatchers.Main) {
                     disconnectPlayer()
                 }
@@ -232,6 +234,8 @@ class PlaybackViewModel
                 currentPlayer.update {
                     PlayerState(player, playerBackend)
                 }
+                configurePlayer()
+            }
         }
 
         private fun configurePlayer() {
@@ -429,7 +433,6 @@ class PlaybackViewModel
 
                 // Create the correct player for the media
                 createPlayer(videoStream?.hdr == true)
-                configurePlayer()
 
                 val subtitleStreams =
                     mediaSource.mediaStreams

app/src/main/java/com/github/damontecres/wholphin/util/mpv/MPVLib.kt

@@ -39,17 +39,9 @@ object MPVLib {
 
     external fun create(appctx: Context)
 
-    fun initialize() {
-        synchronized(this) { init() }
-    }
+    external fun init()
 
-    private external fun init()
-
-    fun tearDown() {
-        synchronized(this) { destroy() }
-    }
-
-    private external fun destroy()
+    external fun destroy()
 
     external fun attachSurface(surface: Surface)
 

app/src/main/java/com/github/damontecres/wholphin/util/mpv/MpvPlayer.kt

@@ -74,6 +74,8 @@ class MpvPlayer(
     SurfaceHolder.Callback {
     companion object {
         private const val DEBUG = false
+
+        private val initLock = Any()
     }
 
     private var surface: Surface? = null
@@ -131,7 +133,7 @@ class MpvPlayer(
         MPVLib.setOptionString("demuxer-max-back-bytes", "${cacheMegs * 1024 * 1024}")
 
         Timber.v("Initializing MPVLib")
-        MPVLib.initialize()
+        MPVLib.init()
 
         MPVLib.setOptionString("force-window", "no")
         MPVLib.setOptionString("idle", "yes")
@@ -1086,17 +1088,21 @@ class MpvPlayer(
             }
 
             MpvCommand.INITIALIZE -> {
+                synchronized(initLock) {
                     init()
                 }
+            }
 
             MpvCommand.DESTROY -> {
-                clearVideoSurfaceView(null)
+                synchronized(initLock) {
+                    MPVLib.setPropertyBoolean("pause", true)
                     MPVLib.removeLogObserver(mpvLogger)
-                MPVLib.tearDown()
+                    MPVLib.destroy()
                     Timber.d("MPVLib destroyed")
                 }
             }
         }
+    }
 }
 
 fun MPVLib.setPropertyColor(