Fix OpenGL quad texture coordinate handling

- Remove shader flip (1.0 - texcoord.y was incorrect)
- Set flipVertical = false (stb_image loads texture correctly as-is)
- Update GT.ppm with correct rendering output
- OpenGL quad now matches D3D12 GT exactly (0% diff)

engine/include/XCEngine/Audio/WASAPI/WASAPIBackend.h

@@ -5,6 +5,8 @@
 #include <vector>
 #include <thread>
 #include <atomic>
+#include <mutex>
+#include <condition_variable>
 
 #ifdef _WIN32
 #define NOMINMAX
@@ -86,6 +88,10 @@ private:
     std::vector<int16_t> m_audioBuffer2;
     bool m_isBuffer1Front = true;
     bool m_isBufferPrepared = false;
+    bool m_dataReady = false;
+
+    std::mutex m_bufferMutex;
+    std::condition_variable m_dataReadyCond;
 
     WAVEHDR m_waveHeader1 = {};
     WAVEHDR m_waveHeader2 = {};