ALSA: Don't block on Clear() call.

snd_pcm_writei() is meant to block block until all samples are written,
but apparently in some situations it can block for much longer, prehaps
even a infinite time, in the case of virtual machine FifoCI runs in.

Because it grabed a mutex before blocking, it could also block the
Clear() call for an infinite length of time, blocking dolphin's emu
thread.

snd_pcm_writei() also takes 10-15 seconds if you run dolphin under GDB
and can randomly take 5 or so seconds during normal usage.

By moving all the pause code to the ALSA thread, Clear() no-longer
blocks and everyone keeps their sanity.

Source/Core/AudioCommon/AlsaSoundStream.cpp

@@ -48,11 +48,10 @@ void AlsaSound::Update()
 void AlsaSound::SoundLoop()
 {
 	Common::SetCurrentThreadName("Audio thread - alsa");
+	while (m_thread_status.load() != ALSAThreadStatus::STOPPING)
+	{
 		while (m_thread_status.load() == ALSAThreadStatus::RUNNING)
 		{
-		std::unique_lock<std::mutex> lock(cv_m);
-		cv.wait(lock, [this]{return !m_muted || m_thread_status.load() != ALSAThreadStatus::RUNNING;});
-
 			m_mixer->Mix(mix_buffer, frames_to_deliver);
 			int rc = snd_pcm_writei(handle, mix_buffer, frames_to_deliver);
 			if (rc == -EPIPE)
@@ -65,6 +64,17 @@ void AlsaSound::SoundLoop()
 				ERROR_LOG(AUDIO, "writei fail: %s", snd_strerror(rc));
 			}
 		}
+		if (m_thread_status.load() == ALSAThreadStatus::PAUSED)
+		{
+			snd_pcm_drop(handle); // Stop sound output
+
+			// Block until thread status changes.
+			std::unique_lock<std::mutex> lock(cv_m);
+			cv.wait(lock, [this]{ return m_thread_status.load() != ALSAThreadStatus::PAUSED; });
+
+			snd_pcm_prepare(handle); // resume sound output
+		}
+	}
 	AlsaShutdown();
 	m_thread_status.store(ALSAThreadStatus::STOPPED);
 }
@@ -73,18 +83,8 @@ void AlsaSound::SoundLoop()
 void AlsaSound::Clear(bool muted)
 {
 	m_muted = muted;
-	if (m_muted)
-	{
-		std::lock_guard<std::mutex> lock(cv_m);
-		snd_pcm_drop(handle);
-	}
-	else
-	{
-		std::unique_lock<std::mutex> lock(cv_m);
-		snd_pcm_prepare(handle);
-		lock.unlock();
-		cv.notify_one();
-	}
+	m_thread_status.store(muted ? ALSAThreadStatus::PAUSED : ALSAThreadStatus::RUNNING);
+	cv.notify_one(); // Notify thread that status has changed
 }
 
 bool AlsaSound::AlsaInit()

Source/Core/AudioCommon/AlsaSoundStream.h

@@ -46,6 +46,7 @@ private:
 	enum class ALSAThreadStatus
 	{
 		RUNNING,
+		PAUSED,
 		STOPPING,
 		STOPPED,
 	};