Add wgc demo

export.cpp

@@ -0,0 +1,17 @@
+#include "pch.h"
+
+#include <winrt/Windows.Foundation.Metadata.h>
+
+bool wgc_is_supported() {
+  try {
+    /* no contract for IGraphicsCaptureItemInterop, verify 10.0.18362.0 */
+    return winrt::Windows::Foundation::Metadata::ApiInformation::
+        IsApiContractPresent(L"Windows.Foundation.UniversalApiContract", 8);
+  } catch (const winrt::hresult_error &) {
+    return false;
+  } catch (...) {
+    return false;
+  }
+}
+
+am::wgc_session *wgc_create_session() { return new am::wgc_session_impl(); }