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I did this because png.h will cause the compile to fail if the user #includes setjmp.h
along with png.h.

Also added interface functions to programmatically observe what is happening to the
overlays.

more than once.

use Release mode by default.

image_display widget.

the default build into a single file.

images of a different size while any kind of zoom was in effect
it could cause a segmentation fault.

jump to the beginning.

routines more robust.

first image.

the XSLT stylesheet file is always created in the same directory as the
main XML metadata file.