Fixes a (small, I think) TMW->Unicode performance glitch. I was

inserting 5 characters at a time and then skipping ahead just one
position.  I don't think this affected correctness.

I believe there's still a terrible (exponential?) slowdown as the
input file gets bigger, however.  Perhaps not -- but we run through
the first 1000 TMW glyphs in 6 seconds, the 20th thousand takes at
least 60 seconds.  Is TMW->Wylie faster than TMW->Unicode?  If so,
why?

Thought: don't use a DuffPane within TibetanConverter -- it can only
add overhead, right?  My hprof profile said that the conversion was
taking just a couple of percent of the work; the rest was going to
display-related stuff that you should only see if you were displaying
the document.  I'm not!

source/org/thdl/tib/text/TibetanDocument.java

@@ -735,6 +735,7 @@ public class TibetanDocument extends DefaultStyledDocument {
                         if (replaceInsteadOfInserting()) {
                             if (toUnicode) {
                                 replaceDuffWithUnicode(fontSize, i, unicode);
+                                i += unicode.length() - 1; // we do i++ below
                             } else {
                                 replaceDuff(fontSize, i, equivalent[0], !toTM);
                             }