Fix issue with .di where .il overrides too many <img> elements

If any .il are in the file, ppgen generates CSS for img to set maxwidth to
100% and height to auto. That causes problems for drop-cap images, which
should not be overridden.

This update makes ppgen generate CSS for .figcenter img, .figleft img, or
.figright img instead, as appropriate for the images present in the source
file.