Prior to this change, hocrtransform would render printable text (black
on white) and then a fully opaque image on top of the text. According to
the PDF spec, text that is the output of OCR should be marked invisible,
so that PDF viewers /know/ it's OCR output in a document that might mix
OCR and text overlays. Another benefit is that PDF viewers would know
to skip rendering text if they are not smart enough to figure out the
image will completely overwrite it.
However, for debug, visible text is nice, so retain it as an option.
Jim Barlow