This implementation renders the entire line segment at once without
subdividing it into scanlines. The main speed improvement comes from
reducing the number of divisions to just two per line segment, which
is a bare minimum to calculate cell coverage in a smooth rasterizer.
Notably, the progression from cell to cell does not itself require any
divisions at all. The speed improvement is more noticeable at larger
sizes.
* src/smooth/ftgrays.c (gray_render_line): New implementation.
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