windows: fix freeze on keyboard layout switch

Any winit app on Windows freezes when the keyboard layout is switched by
tools such as Punto Switcher. A minidump of the frozen process shows the
main thread blocked in MsgWaitForMultipleObjectsEx (winit's event-loop
wait), re-entered during message dispatch through the switcher's injected
global hook (pshook64.dll, via CallNextHookEx) — a re-entrant Win32
message-loop wait, not a mutex deadlock (no LAYOUT_CACHE frames present).

Handle WM_INPUTLANGCHANGE to refresh the cached keyboard layout, then
defer to DefWindowProc so the message still propagates to first-level
child windows as the Win32 docs require. The cache refresh is the minimal
change that stops the freeze (verified on Windows 11 by isolation
variants); update_modifiers and swallowing the message are not needed.