Improve normalization heuristic

Before, for most scripts, we were not trying to recompose two characters
if the second one had ccc=0.  That fails for Myanmar where U+1026
decomposes to U+1025,U+102E, both of which have ccc=0.  However, we do
want to try to recompose those.  We now check whether the second is a
mark, using general category instead.

At the same time, remove optimization that was conflicting with this.

[Let the Ngapi hackfest begin!]