Fix crash when switching from QPainter to openGL canvas for the first time

Yeah, openGL context whould be initialized first.

WARNING: There is a a small bug still.  When switching for the first time,
         the first three changes of the Display Exposure make the canvas
         contain garbage, after that, everything starts working fine. I don't
         know, what it is related to.

         I didn't spend time on that, because it happens *only* when switching
         QPainter->openGL for the first time during a single run of Krita.

CCMAIL:kimageshop@kde.org