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.cpp

@@ -24,6 +24,7 @@
 #include <pxr/base/gf/frustum.h>
 #include <pxr/base/gf/rotation.h>
 
+#include <algorithm>
 #include <cmath>
 #include <unordered_set>
 
@@ -496,6 +497,8 @@ bool UsdSceneRenderer::PickObjectsInRect(
 void UsdSceneRenderer::Render(int width, int height)
 {
     if (!m_stage || width <= 0 || height <= 0) return;
+    m_lastRenderWidth  = width;
+    m_lastRenderHeight = height;
 
     InitRenderer();
     if (!m_renderer) return;
@@ -676,6 +679,40 @@ uint32_t UsdSceneRenderer::GetColorTextureID()
     return att ? static_cast<uint32_t>(att->GetGlTextureName()) : 0;
 }
 
+bool UsdSceneRenderer::CaptureFrame(std::vector<uint8_t>& outRGBA)
+{
+    if (!m_drawTarget || m_lastRenderWidth <= 0 || m_lastRenderHeight <= 0)
+        return false;
+
+    int w = m_lastRenderWidth;
+    int h = m_lastRenderHeight;
+
+    // Resolve MSAA so we read from the non-multisampled colour attachment.
+    m_drawTarget->Resolve();
+
+    outRGBA.resize(static_cast<size_t>(w) * h * 4);
+
+    // Save/restore the read-framebuffer binding so we don't upset ImGui.
+    GLint prevFbo = 0;
+    glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &prevFbo);
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, m_drawTarget->GetFramebufferId());
+    glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, outRGBA.data());
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, static_cast<GLuint>(prevFbo));
+
+    // OpenGL reads bottom-up; flip to top-down.
+    int stride = w * 4;
+    std::vector<uint8_t> row(stride);
+    for (int y = 0; y < h / 2; ++y) {
+        uint8_t* top = outRGBA.data() + y * stride;
+        uint8_t* bot = outRGBA.data() + (h - 1 - y) * stride;
+        std::copy(top, top + stride, row.data());
+        std::copy(bot, bot + stride, top);
+        std::copy(row.data(), row.data() + stride, bot);
+    }
+
+    return true;
+}
+
 // ===========================================================================
 // Axis Overlay (stageView.DrawAxis port)
 // ===========================================================================