xterm-bold is being added when bold font widget differs

[Tyriar]

See microsoft/vscode#29402 (comment)

It seems that if the bold broken logic runs when the terminal is detected from the DOM or not visible it will break it for the life of the terminal as it's only measured once. We should do a better job of re-evaluating broken bold state when the font changes and to redo it later if the terminal is not in the right state.