#46 raster with :sub no longer works

[Scott Johnston]

A raster in a drawtool document file can have a :sub
xb,xe,yb,ye sub-image specification. This is one way
of dealing with large images as described in
http://www.ivtools.org/ivtools/largeimages.html

Haven't tested this in a while, but nowadays there
seems to be a problem with this mechanism. The
sub-image gets rid in, but it is all black.