Draw transparent background in WebGL renderer
Draw transparent background in WebGL renderer

Part of #2252

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