Partially revert d9c53178

This logic changed then timing for when the layer's m_dirtyTexture
was set and unset, which had some side effects. Revert to the old
and known-to-work behavior of using a connection.

Change-Id: I4048e7ae70491afe36b2d766e6c506d9febc44ed
Task-number: QTBUG-41451
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>