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/utils/MovieEncoder.h

@@ -0,0 +1,40 @@
+#pragma once
+#include <string>
+#include <cstdint>
+
+namespace UsdLayerManager {
+
+/// Streams RGBA frames directly into an H.264/MP4 file using libavcodec.
+/// Usage:
+///   MovieEncoder enc;
+///   enc.Open("out.mp4", 1920, 1080, 24.0);
+///   for each frame: enc.WriteFrame(rgbaPixels);   // top-to-bottom, 4 bytes/px
+///   enc.Close();
+class MovieEncoder {
+public:
+    MovieEncoder();
+    ~MovieEncoder();
+
+    // Non-copyable
+    MovieEncoder(const MovieEncoder&) = delete;
+    MovieEncoder& operator=(const MovieEncoder&) = delete;
+
+    bool Open(const char* outputPath, int width, int height, double fps);
+    bool WriteFrame(const uint8_t* rgba);
+    bool Close();   // flush + write trailer; resets state for reuse
+
+    bool IsOpen() const { return m_open; }
+    const std::string& GetLastError() const { return m_error; }
+
+private:
+    bool DrainPackets();
+    void Reset();
+
+    struct Impl;
+    Impl* m_impl = nullptr;
+
+    bool        m_open  = false;
+    std::string m_error;
+};
+
+} // namespace UsdLayerManager