Rate limit Viewport.refresh
Rate limit Viewport.refresh

This prevents 1000 scroll events from firing when the buffer is not full

Fixes #444

xtermjs/xterm.js,cjordog/NRLWebsite,cjordog/NRLWebsite,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,cjordog/NRLWebsite,sourcelair/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,akalipetis/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,cjordog/NRLWebsite,sourcelair/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js