Respect glyph metrics modified by instruction code

Respect glyph metrics modified by instruction code

* src/sfnt.c (sfnt_read_glyph): Clear advance and origin
distortion returning an empty glyph.
(sfnt_build_instructed_outline): New parameter *ADVANCE_WIDTH,
in which the glyph's advance width is saved.
(sfnt_interpret_compound_glyph_1): Refine commentary.
(sfnt_verbose, main): Adjust tests.

* src/sfnt.h: Update prototypes correspondingly.

* src/sfntfont.c (sfntfont_get_glyph_outline): If an instructed
outline is available, derive the advance and lbearing from the
measurements within.
(sfntfont_probe_widths): Call sfntfont_measure_pcm to establish
average widths.
(sfntfont_open): Do so after instruction code initialization
completes.
(sfntfont_measure_pcm): Revise commentary.