#4 zoom doesn't account for margins

[Kevin Conroy]

while in print preview layout, the zoom feature for ink
does not account for the grey margins which retain the
same size regardless of the zoom level.

these margins must be discounted when doing a zoom
(perhaps discount them all together as they should not
really be valid inkable areas... might want to set the
inkable region to only be the white portion of the
document if this is at all possible)