Remove var usage from Viewport
Remove var usage from Viewport

Part of #335

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