Don't transform between QPixmap and QImage all of tghe time without any need.
This fixes a crash reported by Thorsten that
was related to QPixmap not being threadsafe
while we are using libkowmf in the threaded
vectorshape.
It also should make WMF parsing slightly
faster cause we don't transform between
QPixmap and QImage all of the time without
any real need for it.
Tested with cstester.