Add a voice flag to indicate a voice has a pending stop

Pending/asynchronous stops to be implemented soon.

al/source.cpp

@@ -2682,7 +2682,8 @@ START_API_FUNC
     auto count_free_voices = [](const ALuint count, const ALvoice &voice) noexcept -> ALuint
     {
         if(voice.mPlayState.load(std::memory_order_acquire) == ALvoice::Stopped
-            && voice.mSourceID.load(std::memory_order_relaxed) == 0u)
+            && voice.mSourceID.load(std::memory_order_relaxed) == 0u
+            && voice.mPendingStop.load(std::memory_order_relaxed) == false)
             return count + 1;
         return count;
     };
@@ -2760,7 +2761,8 @@ START_API_FUNC
         auto find_voice = [](const ALvoice &v) noexcept -> bool
         {
             return v.mPlayState.load(std::memory_order_acquire) == ALvoice::Stopped
-                && v.mSourceID.load(std::memory_order_relaxed) == 0u;
+                && v.mSourceID.load(std::memory_order_relaxed) == 0u
+                && v.mPendingStop.load(std::memory_order_relaxed) == false;
         };
         auto voices_end = context->mVoices.data() + context->mVoices.size();
         voice = std::find_if(context->mVoices.data(), voices_end, find_voice);

alc/voice.h

@@ -199,10 +199,11 @@ struct ALvoice {
 
     std::atomic<ALvoiceProps*> mUpdate{nullptr};
 
+    ALvoicePropsBase mProps;
+
     std::atomic<ALuint> mSourceID{0u};
     std::atomic<State> mPlayState{Stopped};
-
-    ALvoicePropsBase mProps;
+    std::atomic<bool> mPendingStop{false};
 
     /**
      * Source offset in samples, relative to the currently playing buffer, NOT
@@ -268,11 +269,13 @@ struct ALvoice {
         mUpdate.store(rhs.mUpdate.exchange(old_update, std::memory_order_relaxed),
             std::memory_order_relaxed);
 
+        mProps = rhs.mProps;
+
         mSourceID.store(rhs.mSourceID.load(std::memory_order_relaxed), std::memory_order_relaxed);
         mPlayState.store(rhs.mPlayState.load(std::memory_order_relaxed),
             std::memory_order_relaxed);
-
-        mProps = rhs.mProps;
+        mPendingStop.store(rhs.mPendingStop.load(std::memory_order_relaxed),
+            std::memory_order_relaxed);
 
         mPosition.store(rhs.mPosition.load(std::memory_order_relaxed), std::memory_order_relaxed);
         mPositionFrac.store(rhs.mPositionFrac.load(std::memory_order_relaxed),