Redo the calculation of the bounding box in a better way.

The new way means that the bounding box is only recalculated when
there is a drawing operation. This means that changes to the
window/viewport origin or extension with no drawing are
disregarded. It also means that there is no recalculation when both
org and ext are changed in between but just after both have changed.

This probably means that we get a much better bounding box in many
cases.