Platform: update WindowlessWindowsEglContext to match the WglContext.

Applied 78ddfad543 on this class. Also
fixed suspicious window creation -- the private member variable was
never initialized.

src/Magnum/Platform/WindowlessWindowsEglApplication.cpp

@@ -36,25 +36,34 @@
 namespace Magnum { namespace Platform {
 
 WindowlessWindowsEglContext::WindowlessWindowsEglContext(const Configuration& configuration, Context*) {
-    /* Create the window */
+    /* Register the window class (if not yet done) */
-    const WNDCLASS wc{
+    WNDCLASSW wc;
+    if(!GetClassInfoW(GetModuleHandleW(nullptr), L"Magnum Windowless Application", &wc)) {
+        wc = WNDCLASSW{
             0,
-        DefWindowProc,
+            DefWindowProcW,
             0,
             0,
-        GetModuleHandle(nullptr),
+            GetModuleHandleW(nullptr),
             nullptr,
             nullptr,
             HBRUSH(COLOR_BACKGROUND),
             nullptr,
             L"Magnum Windowless Application"
         };
-    if(!RegisterClass(&wc)) return;
+
-    const HWND window = CreateWindowW(wc.lpszClassName, L"Magnum Windowless Application",
+        if(!RegisterClassW(&wc)) {
+            Error() << "Platform::WindowlessWglContext: cannot create window class:" << GetLastError();
+            return;
+        }
+    }
+
+    /* Create the window */
+    _window = CreateWindowW(wc.lpszClassName, L"Magnum Windowless Application",
         WS_OVERLAPPEDWINDOW, 0, 0, 32, 32, 0, 0, wc.hInstance, 0);
 
     /* Initialize */
-    _display = eglGetDisplay(GetDC(window));
+    _display = eglGetDisplay(GetDC(_window));
     if(!eglInitialize(_display, nullptr, nullptr)) {
         Error() << "Platform::WindowlessWindowsEglApplication::tryCreateContext(): cannot initialize EGL:" << Implementation::eglErrorString(eglGetError());
         return false;