Just after showing (i.e. in showEvent), contentsRect() width (or height)
is 0, which after calculations leads to _usedColumns (or _usedLines)
being equal to 1. Valid contentsRect() is restored just after showEvent,
and is available in resizeEvent.

Test:
* Split left/right
* Maximize one terminal
* Unmaximize

Without patch:
Previously hidden terminal appears with only one column painted

With patch:
Normal, fully painted terminal is shown.