fix looking up wrong word in reflowed page
that has a text layer underneath.

For multi-column page reflowing usually fits individual column to
page width and no finner word boxes are available to map reflowed
word box onto native word box. Now the relative position of a reflowed
word center is calculated as portion of the width and height of a larger
block it belongs. This relative position is also considered in
reflow-to-native position transform so that dictionary lookup now works
on multi-column pages in reflowing mode.

This should fix #376.