If any client currently displaying a window pane has more than 1 KB of output

buffered, don't accept any further data from the process running in the pane.

This makes tmux much more responsive when flooded with output, although other
buffers can still have an impact when running remotely.

Prompted by a query from Ranganathan Sankaralingam.

tmux.h

@@ -65,6 +65,9 @@ extern char   **environ;
 /* Maximum poll timeout (when attached). */
 #define POLL_TIMEOUT 50
 
+/* Maximum data to buffer for output before suspending reading from panes. */
+#define BACKOFF_THRESHOLD 1024
+
 /*
  * Maximum sizes of strings in message data. Don't forget to bump
  * PROTOCOL_VERSION if any of these change!