Do not paint long pre-edit text outside display

Summary: {F5822474}

Test Plan:
* Turn on IME (tested with ibus+anthy)
* Type (without accepting) long text using IME

Expected result: text should not be drawn outside terminal
Actual result: text is drawn on terminal, margins, scrollbar

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12556