Don't override decoration positions
Don't override decoration positions

This also fixes an exception that was throwing where we only allowed links
within the terminal's 'rows', which should have been buffer length

Fixes #3705
Part of microsoft/vscode#145808

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