CheckBox & RadioButton: fix rounding issue that caused elided text

Task-number: QTBUG-39271
Change-Id: I9ddf03e2274f1abf4e049b84c3f72050f22cc958
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>