Windows Pageant: fix hang due to queued callbacks.

In the Windows Pageant message loop, we were alternating between
MsgWaitForMultipleObjects with timeout=INFINITE, and
run_toplevel_callbacks. But run_toplevel_callbacks doesn't loop until
the callback queue is empty: it just runs at least one of the
currently queued callbacks, so that some progress was made. So if two
or more callbacks were queued, we'd leave the rest in the queue and go
back into MsgWaitForMultipleObjects, which could hang indefinitely.

(A very silly workaround was available: move the mouse over the
Pageant systray icon! Every mouse event would terminate the wait and
let you get one more iteration of run_toplevel_callbacks.)

Now we do the same thing as in other main loops: if any further
callbacks are pending, then we still run MsgWaitForMultipleObjects,
but we do it with timeout=0 instead of timeout=INFINITE, so that we
won't go back to a _blocking_ sleep until all callbacks have been
serviced.

windows/pageant.c

@@ -1648,8 +1648,9 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 
         HandleWaitList *hwl = get_handle_wait_list();
 
+        DWORD timeout = toplevel_callback_pending() ? 0 : INFINITE;
         n = MsgWaitForMultipleObjects(hwl->nhandles, hwl->handles, false,
-                                      INFINITE, QS_ALLINPUT);
+                                      timeout, QS_ALLINPUT);
 
         if ((unsigned)(n - WAIT_OBJECT_0) < (unsigned)hwl->nhandles)
             handle_wait_activate(hwl, n - WAIT_OBJECT_0);