Bug 707755 - reviewing PNG comment saving algorithm

Currently PNG "comment" is saved in iTXt (UTF-8) if supported, tEXt
(ISO-8859-1) otherwise. The problem is that some software out there like
ImageMagick would apparently only read tEXt comments.
Therefore the replacing algorithm is:
1/ if we would not lose any character in a conversion from UTF-8 to
ISO-8859-1, we save in tEXt, whether or not the platform supports iTXt.
2/ if we would lose comment data in the conversion while iTXt is
supported, we save in iTXt.
3/ if iTXt is not supported, we save in tEXt anyway and discard any
non-convertible character, unless the finale result is an empty string
(in which case, we don't save any comment).
plug-ins - compress the PNG comment when over a given size.