Prevent erase_when_done affecting full screen apps

[joouha]

If erase_when_done and full_screen are both set to True for an application, then the renderer never gets reset. This causes issues such as the cursor does not getting shown if it is hidden when exiting an application.

This PR works around this by ignoring erase_when_done for full-screen apps.

I think erase_when_done should not really apply to full screen apps, as they effectively will always get erased when exiting the alternate screen.

I think it's unlikely that anyone depends on the existing behaviour, so hopefully it wouldn't break anything for anyone - let me know what you think.

[jonathanslenders]

I think we should call output.show_cursor() in Renderer.erase(), similar to to the way we reset styling attributes and autowrap.