Introduce the rudiments of a new font rasterizer

Introduce the rudiments of a new font rasterizer

* src/sfnt.c (xzalloc): New function.
(sfnt_poly_edges): Remove redundant statement.
(sfnt_add, sfnt_sub, sfnt_mul): Move macro definitions above the
new scaler.
(sfnt_build_outline_fedges, sfnt_insert_raster_step)
(sfnt_fedge_sort, sfnt_poly_edges_exact, sfnt_compute_fill)
(sfnt_poly_steps, sfnt_raster_steps, sfnt_raster_edges_exact)
(sfnt_raster_glyph_outline_exact): New functions, presently
disabled.
(sfnt_x_raster, main): Introduce new tests.

(cherry picked from commit 6f14fc5f3d4f6de489de3079f3dc3ff486b270c6)