Fix non-reproducible TrueType hinting: invalid qsort comparators

Fix non-reproducible TrueType hinting: invalid qsort comparators

Several comparison functions passed to qsort() violated the ordering
axioms. With glibc >= 2.37 qsort() takes an in-place introsort path that
has undefined behaviour for such comparators and reads uninitialised
memory, so the generated TrueType instructions varied between builds
(e.g. FreeSerif.ttf from the GNU FreeFont sources).

Make SortBlues(), SortStems(), sortbynum(), sortedges(), stem_cmp() and
line_pt_cmp() return a proper trichotomous result and compare both
arguments symmetrically.

Gbp-Pq: Name 0004-qsort-comparators.patch