Resizing a terminal that is not being displayed will break the viewport/scroll area

[Tyriar]

See microsoft/vscode#18332

This is happening because the terminal container is being hidden which causes CharMeasure to return 0 which puts the viewport into an invalid state. There should be a reasonable way of doing this to prevent obscure bugs like above.

[Tyriar]

Reliable demo repro:

1. Fill buffer
2. In dev tools add display:none to terminal-container
3. In dev tools run term.resize(80, 15)
4. Remove display:none style, the viewport will be messed up until another resize occurs