Cache a copy of the active buffer as a private prop
Cache a copy of the active buffer as a private prop

This reduces GC from the const buffer workaround that avoids excessive getter access
with far less getter access

Part of #3450

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