Reset visible area when RIS is received
When running bash and executing `echo -ne '\033c'`, the terminal should be cleared. However there was an issue with the visible area not being cleared, so all the text previously printed would still remain visible. To fix this, whenever a `reset` call is received now, the complete visible area is reset to `Cell::default()` (the default Cell) and the length of the available scrollback history is reset to `0`, which results in the scrollback history being cleared from the perspective of the user. This fixes #1483.
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