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>

CMakeLists.txt

@@ -14,6 +14,7 @@ find_package(OpenGL REQUIRED)
 find_package(OpenUSD REQUIRED)
 find_package(Imgui REQUIRED)
 find_package(Glad REQUIRED)
+find_package(FFmpeg REQUIRED)
 
 # USD build that includes hdEmbree (built with PXR_ENABLE_EMBREE_PLUGIN=ON).
 # Can be a separate install from OpenUSD_ROOT_DIR; leave empty to search there instead.
@@ -177,6 +178,13 @@ target_link_libraries(UsdLayerManager PRIVATE
     OpenUSD::OpenUSD
     Imgui::Imgui
     Glad::Glad
+    FFmpeg::FFmpeg
+    ole32
+    shell32
+)
+
+target_include_directories(UsdLayerManager PRIVATE
+    "${CMAKE_SOURCE_DIR}/third_party/ffmpeg/include"
 )
 
 # Cycles is an ExternalProject; UsdLayerManager must wait for it to finish
@@ -194,6 +202,17 @@ if(WIN32)
         _CRT_SECURE_NO_WARNINGS
     )
     
+    # FFmpeg DLLs — copy all .dll files from third_party/ffmpeg/bin
+    file(GLOB FFMPEG_RUNTIME_DLLS "${CMAKE_SOURCE_DIR}/third_party/ffmpeg/bin/*.dll")
+    if(FFMPEG_RUNTIME_DLLS)
+        add_custom_command(TARGET UsdLayerManager POST_BUILD
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                ${FFMPEG_RUNTIME_DLLS}
+                "$<TARGET_FILE_DIR:UsdLayerManager>"
+            COMMENT "Copying FFmpeg runtime DLLs..."
+        )
+    endif()
+
     # Copy runtime DLLs to output directory for each configuration
     add_custom_command(TARGET UsdLayerManager POST_BUILD
         COMMAND ${CMAKE_COMMAND} -E make_directory "$<TARGET_FILE_DIR:UsdLayerManager>"
@@ -504,11 +523,13 @@ add_executable(ViewportDisplayTest
 target_include_directories(ViewportDisplayTest PRIVATE
     ${CMAKE_SOURCE_DIR}/src
     ${CMAKE_SOURCE_DIR}/include
+    "${CMAKE_SOURCE_DIR}/third_party/ffmpeg/include"
 )
 
 target_link_libraries(ViewportDisplayTest PRIVATE
     OpenUSD::OpenUSD
     Glad::Glad
+    FFmpeg::FFmpeg
 )
 
 if(WIN32)
@@ -550,11 +571,13 @@ add_executable(RendererDiagnosticTest
 target_include_directories(RendererDiagnosticTest PRIVATE
     ${CMAKE_SOURCE_DIR}/src
     ${CMAKE_SOURCE_DIR}/include
+    "${CMAKE_SOURCE_DIR}/third_party/ffmpeg/include"
 )
 
 target_link_libraries(RendererDiagnosticTest PRIVATE
     OpenUSD::OpenUSD
     Glad::Glad
+    FFmpeg::FFmpeg
 )
 
 if(WIN32)