[sfnt] Support `face->num_faces' for WOFF2 fonts.

Set correct value of `face->num_faces' for WOFF2 fonts.  This is
being handled separately because we only load the tables for the
requested font face in `woff2_open_font' and create a single-face
sfnt stream.

The full discussion is at:


* src/sfnt/sfobjs.c (sfnt_open_font): Add parameter
(sfnt_init_face): Introduce variable `woff2_num_faces', and change
`face->root.num_faces' if `woff2_num_faces' is set.

* src/sfnt/sfwoff2.c (woff2_open_font): Validate requested face
index and handle negative face indices.

* src/sfnt/sfwoff2.h (woff2_open_font): Add parameter `num_faces' to
4 files changed