Fixed bug where an SDL window that was activated while hidden could never be shown.

Test code:
{
	SDL_Window *win = SDL_CreateWindow( "Dummy", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 128, 128, SDL_WINDOW_HIDDEN );
	SDL_SysWMinfo info;
	SDL_VERSION( &info.version );
	SDL_GetWindowWMInfo( win, &info );
	SetActiveWindow( info.info.win.window );
	{
		DWORD then = SDL_GetTicks();
		while ( ( SDL_GetTicks() - then ) < 3000 )
		{
			SDL_Event evt;
			SDL_PollEvent( &evt );
		}
		SDL_ShowWindow( win );

		then = SDL_GetTicks();
		while ( ( SDL_GetTicks() - then ) < 3000 )
		{
			SDL_Event evt;
			SDL_PollEvent( &evt );
		}
	}
	SDL_DestroyWindow( win );
}

src/video/windows/SDL_windowsevents.c

@@ -421,6 +421,11 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
             POINT cursorPos;
             BOOL minimized;
 
+            /* Don't mark the window as shown if it's activated before being shown */
+            if (!IsWindowVisible(hwnd)) {
+                break;
+            }
+
             minimized = HIWORD(wParam);
             if (!minimized && (LOWORD(wParam) != WA_INACTIVE)) {
                 if (LOWORD(wParam) == WA_CLICKACTIVE) {