Fix corrupted MP4 recordings on Windows by serializing encoder writes

The WGC capture session uses Direct3D11CaptureFramePool::CreateFreeThreaded,
which dispatches FrameArrived callbacks on thread pool threads concurrently.
MFEncoder::writeFrame() accesses non-thread-safe resources (ID3D11DeviceContext,
IMFSinkWriter, shared staging texture and NV12 buffer) without synchronization,
causing MP4 container corruption when callbacks overlap at high frame rates.

Add a mutex to serialize writeFrame() and finalize() calls, preventing
concurrent access to the D3D context and Media Foundation sink writer.

Fixes #96

electron/native/wgc-capture/src/mf_encoder.cpp

@@ -124,6 +124,7 @@ bool MFEncoder::initialize(const std::wstring& outputPath, int width, int height
 }
 
 bool MFEncoder::writeFrame(ID3D11Texture2D* texture, int64_t timestampHns) {
+    std::lock_guard<std::mutex> lock(writeMutex_);
     if (!initialized_ || !sinkWriter_) return false;
 
     context_->CopyResource(stagingTexture_.Get(), texture);
@@ -190,6 +191,7 @@ bool MFEncoder::writeFrame(ID3D11Texture2D* texture, int64_t timestampHns) {
 }
 
 bool MFEncoder::finalize() {
+    std::lock_guard<std::mutex> lock(writeMutex_);
     if (!initialized_) return false;
     initialized_ = false;
 

electron/native/wgc-capture/src/mf_encoder.h

@@ -8,6 +8,7 @@
 #include <wrl/client.h>
 #include <string>
 #include <vector>
+#include <mutex>
 
 using Microsoft::WRL::ComPtr;
 
@@ -32,4 +33,5 @@ private:
     int height_ = 0;
     int fps_ = 60;
     bool initialized_ = false;
+    std::mutex writeMutex_;
 };