Apply latest consecutive resize, not earliest.

If there are consecutive resize events in the userstream to be applied in
"serve", we should apply the last/latest one in the sequence, not the
first/earliest one.

This fixes a problem where a flurry of resize events (eg, generated
by a window manager resizing the client), can cause mosh to have an
out-of-date idea as to what the physical geometry of the window is.