[smooth] Faster bitmap sweeping.

Selecting the fill rule or checking the direct mode each time we call
`gray_hline' is sub-optimal.  This effectively splits the direct mode
into a separate code path while inlining `gray_hline' and saving 5-7%
of rendering time.

* src/smooth/ftgrays.c (gray_hline): Eliminated in favor of...
(FT_FILL_RULE, FT_GRAY_SET): ... these new macros...
(gray_sweep): ... inlined here.
(gray_sweep_direct): New function that handles the direct span buffer.
(gray_TWorker): Remove the span buffer.
(gray_raster_render, gray_convert_glyph): Updated.
2 files changed