Introduce a new _overflowed state variable to indicate whether the
horizontal boundary was reached already and to omit subsequent character
output.
This state is necessary to maintain a valid cursor position at all
times. The _overflowed attribute is reset once the cursor is moved into
a valid position again.
To harmonise the bounds checking for _cursor_pos modifications, the
`constrain()` method was added.
Fixesgenodelabs/genode#4093
Also fixes a bug in `vpa()` and `vpb()` which moved the cursor
horizontally instead of vertically.
1c49da8ce4