[XMA] Fix dedicated thread stalls by blocking guest audio thread

Includes various other thread safety and timing related adjustments

src/xenia/apu/xma_context.h

@@ -15,6 +15,7 @@
 #include <mutex>
 #include <queue>
 
+#include "xenia/base/threading.h"
 #include "xenia/memory.h"
 #include "xenia/xbox.h"
 
@@ -212,11 +213,28 @@ class XmaContext {
 
   uint32_t id() { return id_; }
   uint32_t guest_ptr() { return guest_ptr_; }
-  bool is_allocated() { return is_allocated_; }
-  bool is_enabled() { return is_enabled_; }
+  bool is_allocated() { return is_allocated_.load(std::memory_order_acquire); }
+  bool is_enabled() { return is_enabled_.load(std::memory_order_acquire); }
 
-  void set_is_allocated(bool is_allocated) { is_allocated_ = is_allocated; }
-  void set_is_enabled(bool is_enabled) { is_enabled_ = is_enabled; }
+  void set_is_allocated(bool is_allocated) {
+    is_allocated_.store(is_allocated, std::memory_order_release);
+  }
+  void set_is_enabled(bool is_enabled) {
+    is_enabled_.store(is_enabled, std::memory_order_release);
+  }
+
+  // Signals that the worker has finished processing this context after a kick.
+  void SignalWorkDone() {
+    if (work_completion_event_) {
+      work_completion_event_->Set();
+    }
+  }
+  // Blocks until the worker has finished processing this context.
+  void WaitForWorkDone() {
+    if (work_completion_event_) {
+      xe::threading::Wait(work_completion_event_.get(), false);
+    }
+  }
 
  protected:
   static void DumpRaw(AVFrame* frame, int id);
@@ -229,8 +247,9 @@ class XmaContext {
   uint32_t id_ = 0;
   uint32_t guest_ptr_ = 0;
   xe_mutex lock_;
-  volatile bool is_allocated_ = false;
-  volatile bool is_enabled_ = false;
+  std::atomic<bool> is_allocated_ = false;
+  std::atomic<bool> is_enabled_ = false;
+  std::unique_ptr<xe::threading::Event> work_completion_event_;
 
   // ffmpeg structures
   AVPacket* av_packet_ = nullptr;