imglab annotation format.

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.