Fixed minimized window detection when handling WM_WINDOWPOSCHANGED

When minimizing a window, we get this sequence of events:
WM_WINDOWPOSCHANGING
WM_GETMINMAXINFO
WM_NCCALCSIZE
WM_WINDOWPOSCHANGED - IsIconic() is true
WM_MOVE
WM_SIZE - SDL sees minimized state here

When restoring a window, we get this sequence of events:
WM_WINDOWPOSCHANGING
WM_GETMINMAXINFO
WM_NCCALCSIZE
WM_NCPAINT
WM_ERASEBKGND
WM_WINDOWPOSCHANGED - IsIconic() is false
WM_MOVE
WM_SIZE - SDL sees restored state here

On Windows 10 a minimized window has a non-empty client rect, so we were delivering a minimized size before SDL knows that the window is minimized, and then ignoring the restored size when handling the restore message.

The fix is to use IsIconic() which returns the correct window state when WM_WINDOWPOSCHANGED is actually delivered.

src/video/windows/SDL_windowsevents.c

@@ -1169,7 +1169,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
             }
 
             /* When the window is minimized it's resized to the dock icon size, ignore this */
-            if ((data->window->flags & SDL_WINDOW_MINIMIZED) != 0) {
+            if (IsIconic(hwnd)) {
                 break;
             }