Platform: avoid registering WNDCLASS more than once.

Fixes an issue when creating the context more than once in a single
application. Also changed the API calls to be explicitly "W" and added
proper error reporting to this part of code.

src/Magnum/Platform/WindowlessWglApplication.cpp

@@ -46,20 +46,29 @@
 namespace Magnum { namespace Platform {
 
 WindowlessWglContext::WindowlessWglContext(const Configuration& configuration, Context* const magnumContext) {
-    /* 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;
+
+        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);