[GREENFOOT-721] Export screen snapshot selector issues

The control for selecting the snapshot image for an exported (to web) scenario has several issues (found with Ants scenario):

outside world edge can be dragged past selector edge, i.e. it is possible to drag the world image so that part of the snapshot is blank, even though the world image at the current zoom level is larger than the snapshot size. It is even possible to drag the world image completely out of the snapshot area.
zooming out does not correctly clear the area outside the new world size, leaving artifacts
It is possible to zoom in to the point that the world image is slighty smaller in both dimensions than the snapshot size. This should be possible only for one dimension.

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