Properly parse format 4 cmap tables

Properly parse format 4 cmap tables

* src/sfnt.c (sfnt_read_cmap_format_4): Read range_shift field
propery.  Prior to this, it would be inadvertently treated as an
entry within the segment end code array, which only functioned
by happenstance.
(sfnt_lookup_glyph_4): Remove workaround grounded upon an
erroneous interpretation of the bug fixed by the aformentioned
change.

* src/sfnt.h (struct sfnt_cmap_format_4): Introduce previously
absent `range_shift' field.