Fix mid mouse button click on the scrollbar

On 45dc1990 i made the wrong assumption that if you were in
slotRequestVisiblePixmaps with the mid mouse button pressed
it meant that you were mid mouse button zooming, but this is not
true if you mid mouse click on the scrollbar.

So the solution to ignore pixmap request on mid mouse button zooming
is actually use blockPixmapsRequest

BUGS: 341583
FIXED-IN: 14.12.1