Enable backend switching for Windows Viewer

tools/viewer/sk_app/win/Window_win.cpp

Index: tools/viewer/sk_app/win/Window_win.cpp
diff --git a/tools/viewer/sk_app/win/Window_win.cpp b/tools/viewer/sk_app/win/Window_win.cpp
index e9905d42cced451cb57b867e7ab15dbcdf89742c..478c81f1d47df39f4e0fd7b8f31ddd31db4ec071 100644
--- a/tools/viewer/sk_app/win/Window_win.cpp
+++ b/tools/viewer/sk_app/win/Window_win.cpp
@@ -33,29 +33,34 @@ Window* Window::CreateNativeWindow(void* platformData) {
 
 LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
 
+
 bool Window_win::init(HINSTANCE hInstance) {
     fHInstance = hInstance ? hInstance : GetModuleHandle(nullptr);
 
-    WNDCLASSEX wcex;
     // The main window class name
     static const TCHAR gSZWindowClass[] = _T("SkiaApp");
 
-    wcex.cbSize = sizeof(WNDCLASSEX);
-
-    wcex.style = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
-    wcex.lpfnWndProc = WndProc;
-    wcex.cbClsExtra = 0;
-    wcex.cbWndExtra = 0;
-    wcex.hInstance = fHInstance;
-    wcex.hIcon = LoadIcon(fHInstance, (LPCTSTR)IDI_WINLOGO);
-    wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);;
-    wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
-    wcex.lpszMenuName = nullptr;
-    wcex.lpszClassName = gSZWindowClass;
-    wcex.hIconSm = LoadIcon(fHInstance, (LPCTSTR)IDI_WINLOGO);;
-
-    if (!RegisterClassEx(&wcex)) {
-        return false;
+    static WNDCLASSEX wcex;
+    static bool wcexInit = false;
+    if (!wcexInit) {
+        wcex.cbSize = sizeof(WNDCLASSEX);
+
+        wcex.style = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
+        wcex.lpfnWndProc = WndProc;
+        wcex.cbClsExtra = 0;
+        wcex.cbWndExtra = 0;
+        wcex.hInstance = fHInstance;
+        wcex.hIcon = LoadIcon(fHInstance, (LPCTSTR)IDI_WINLOGO);
+        wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);;
+        wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
+        wcex.lpszMenuName = nullptr;
+        wcex.lpszClassName = gSZWindowClass;
+        wcex.hIconSm = LoadIcon(fHInstance, (LPCTSTR)IDI_WINLOGO);;
+
+        if (!RegisterClassEx(&wcex)) {
+            return false;
+        }
+        wcexInit = true;
     }
 
    /*
@@ -175,7 +180,6 @@ LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
             break;
 
         case WM_CLOSE:
-        case WM_DESTROY:
             PostQuitMessage(0);
             eventHandled = true;
             break;