[smooth] Reduce copying during integration phase.

We now record `cover' and `area' directly into the linked list. This
makes rendering faster by 10% or even more at larger sizes.

* src/smooth/ftgrays.c (FT_INTEGRATE): Write directly.
(gray_TWorker): Add direct cell reference and remove unused fields.
(gray_set_cell): Consolidate the linked list management and pointers.
(gray_convert_glyph, gray_convert_glyph_inner): Updated.
2 files changed