Support min contrast ratio in canvas renderer too:
Support min contrast ratio in canvas renderer too:

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