It's important for us to take maring into account because rouding error
can force us to skip right marging. However, this shouldn't be done
when not in pan_by_page mode, so this diff is artifically large
re-apply the changes introduced by HW in commit:
3c56f50cae
I mistakenly overwrote it when merging from djvu
branch. :(
Since the title needs to be cleaned in two places,
I turn it into a method.
NOTE that the jumpstack maintaining strategy has
been changed in this commit compared to commit #15
and #31. Now current viewing page in kept in the
stack to preserve the notes.
Add unireader class for better code reuse.
Now pdfreader and djvureader are subclass of
unireader.
Every subclass needs to have a init() method
that will be called in reader.lua.
