wb: hyphenate, insert ligatures and kern after each subword

The word buffer now collects consecutive characters (subwords).  When
a subword ends, the word buffer finds the hyphenation points, inserts
possible ligatures, and performs pairwise kerning for the whole
subword.  This allows more complex text processing; now it is possible
to implement more complex OTF features as described in its GSUB and
GPOS tables.