Implement pImpl pattern for KRContext/Context, fix crash in KRDataBlock

tests/smoke/hello_cube/main_win.cpp

@@ -7,6 +7,8 @@ LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
 
 int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
 {
+  Context* context = Context::Get();
+
   MSG msg = { 0 };
   WNDCLASS wc = { 0 };
   wc.lpfnWndProc = WndProc;