Add playblast: viewport capture to H.264 MP4 via libavcodec

- TimelinePanel: Playblast button opens modal with output dir (native
  folder picker via IFileOpenDialog), custom resolution (HD/FHD/4K
  presets), frame range, and movie export toggle
- UsdSceneRenderer: CaptureFrame() reads pixels from resolved MSAA FBO;
  Render() stores last dimensions for off-screen re-render at capture res
- MovieEncoder: streams RGBA frames directly into MP4 using libavcodec/
  libswscale (H.264, tries libx264 → nvenc → qsv → amf → openh264)
- Application: CapturePlayblastFrame() re-renders at capture resolution
  each frame; opens/writes/closes MovieEncoder inline with the render loop
- FileDialog: BrowseFolder() using Vista-style IFileOpenDialog (COM)
- CMake: FindFFmpeg.cmake locates prebuilt BtbN GPL shared package in
  third_party/ffmpeg; links and copies DLLs for main and test targets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/core/UsdSceneRenderer.h

@@ -212,10 +212,20 @@ public:
     // -----------------------------------------------------------------------
     uint32_t GetColorTextureID();
 
+    /// Dimensions of the most recent Render() call.
+    int GetLastRenderWidth()  const { return m_lastRenderWidth;  }
+    int GetLastRenderHeight() const { return m_lastRenderHeight; }
+
+    /// Read the last rendered frame into top-down RGBA8 pixels.
+    /// Resolves MSAA before reading. Returns false if the draw target is not ready.
+    bool CaptureFrame(std::vector<uint8_t>& outRGBA);
+
     // For unit tests
     pxr::GlfDrawTargetRefPtr GetDrawTargetForTest() const { return m_drawTarget; }
 
 private:
+    int m_lastRenderWidth  = 0;
+    int m_lastRenderHeight = 0;
     void InitRenderer();
     void InitGridResources();
     void RebuildGridVBO();          ///< (Re)build grid line geometry after up-axis or size change.