Do not delete m_blitProgram more than once

When m_blitProgram is created the constructor will add it to
QOpenGLContext as a child of that QObject, so it will be deleted
when that QOpenGLContext is deleted as QObject will delete its
children in destructor. Delete it here will cause crash in
thread termination.

Change-Id: If9d3287d159cc3276b6a840a584a1b212b9c9fd0
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>