When the data we have buffered to write to a terminal grows beyond a

When the data we have buffered to write to a terminal grows beyond a
reasonable amount (currently width * height * 8 bytes), discard all
output to the terminal and start trying to redraw periodically
instead. Continue with this until the amount of data we are trying to
write falls to a low level again.

This helps to prevent tmux sitting on a huge buffer of data when there
are processes with fast output running inside tmux but the outside
terminal is slow.

A new client_discarded format holds the amount of data that has been
discarded due to this mechanism.

The three variables (when to start this, when to stop, and how often to
redraw) are basically "works for me" at the moment, this is going in to
see how it goes and if it causes problems for anyone else.