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 );
}

     1.1 --- a/src/video/windows/SDL_windowsevents.c	Sat Mar 24 10:26:40 2018 -0700
     1.2 +++ b/src/video/windows/SDL_windowsevents.c	Mon Mar 26 12:38:29 2018 -0700
     1.3 @@ -421,6 +421,11 @@
     1.4              POINT cursorPos;
     1.5              BOOL minimized;
     1.6  
     1.7 +            /* Don't mark the window as shown if it's activated before being shown */
     1.8 +            if (!IsWindowVisible(hwnd)) {
     1.9 +                break;
    1.10 +            }
    1.11 +
    1.12              minimized = HIWORD(wParam);
    1.13              if (!minimized && (LOWORD(wParam) != WA_INACTIVE)) {
    1.14                  if (LOWORD(wParam) == WA_CLICKACTIVE) {