Not all possible keyboard modifiers of a wheel event change
MagnifyingGlass's pixmap. So in case of e.g. MetaModifier or more than
one modifier, MagnifyingGlass::wheelEvent() calls updateImage() to no
avail.
Move the updateImage() call into the public slots to make them
self-sufficient. This also allows not to call updateImage() when the
pixmap is not changed in case the adjusted parameter has reached its
minimum or maximum value already.
const QPixmap should perform as fast as a pointer
there is a replacement version for Qt5 but it requires Qt5.15, so we are using indirection witch is unsafer but it should be ok as all the action is happen in the main thread
