Only sync viewport in an animation frame
Only sync viewport in an animation frame

Fixes #444

xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js