Create audio decoders asychronously

To avoid initial file read from disk in the main thread. Use deadline to avoid
noticable delay in actors speech start. This changed affects only voices and
music. Sounds are not affected by this change.

Decoder initialization requires to open file to read headers that leads to
read from disk. This operation is performed by a separated thread via
SceneUtil::WorkQueue. Created and initialized decoders are stored into
a map file name -> decoder. Other data required to play voice stored into
SoundManager state and used each update from the main thread to try to start
playing voices and music when corresponding decoder is available. If it's
not available play is delayed until next frame. When deadline to create
decoder is reached main thread stops and waits until that decoder is created.

apps/openmw/mwsound/sound.hpp

@@ -38,6 +38,15 @@ namespace MWSound
 
         SoundParams mParams;
 
+        enum class State
+        {
+            Loading,
+            Playing,
+            LoadCancelled,
+        };
+
+        State mState = State::Loading;
+
     protected:
         Sound_Instance mHandle = nullptr;
 
@@ -58,6 +67,8 @@ namespace MWSound
                 mParams.mFadeOutTime -= soundDuration;
             }
         }
+        void setPlaying() { mState = State::Playing; }
+        void cancelLoading() { mState = State::LoadCancelled; }
 
         const osg::Vec3f &getPosition() const { return mParams.mPos; }
         float getRealVolume() const { return mParams.mVolumeFactor * mParams.mSfxVolume * mParams.mBaseVolume; }
@@ -75,6 +86,7 @@ namespace MWSound
         void init(const SoundParams& params)
         {
             mParams = params;
+            mState = State::Loading;
             mHandle = nullptr;
         }