[truetype] Clean up phantom point accounting.

This formalizes that the phantom points appended in the outline
do not increase its point count, nor are they tagged or included
in any additional contours.  Only their coordinates are stored.
They are counted in the glyph zone, however.

* src/truetype/ttgxvar.c (TT_Vary_Apply_Glyph_Deltas): Infer `n_points`
from the outline size plus four phantom points.
* src/truetype/ttgxvar.h (TT_Vary_Apply_Glyph_Deltas): Remove this
* src/truetype/ttgload.c (tt_prepare_zone): Add phantom four.
(TT_Process_Simple_Glyph, load_truetype_glyph): Update all callers.
3 files changed