Prevent dragging from setting selection y over bounds
Prevent dragging from setting selection y over bounds

Fixes #1152

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