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/ui/Application.h

@@ -11,6 +11,7 @@
 #include "../core/LayerManager.h"
 #include "../core/PropertyManager.h"
 #include "../core/CommandHistory.h"
+#include "../utils/MovieEncoder.h"
 #include <memory>
 #include <string>
 
@@ -43,6 +44,9 @@ private:
     // Stage editing operations (also exposed via Stage menu)
     void AddReferenceToStage();
     void CreatePrimOnStage(const std::string& typeName);
+
+    // Playblast — called each frame while a capture is in progress.
+    void CapturePlayblastFrame();
     
     std::unique_ptr<ImGuiContext> m_imguiContext;
     std::unique_ptr<IconManager> m_iconManager;
@@ -63,6 +67,8 @@ private:
     bool m_showPropertyPanel;
     bool m_showTimeline;
     bool m_running;
+
+    MovieEncoder m_movieEncoder;
 };
 
 } // namespace UsdLayerManager