Lower z-indexes
Lower z-indexes

Canvas layers: 0-4
Helpers: 5
A11y tree: 10

Fixes #2275

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