[Pixman] [PATCH 1/4] Implement basic dithering for the wide pipeline, v3

[basile-pixman at clement.pm]

v2 was disabling the `linear_gradient_is_horizontal` optimization when
the wide pipeline is used because a very old version of this patch was
performing dithering in `linear_get_scanline`, which required it to be
recomputed for each scanline even for horizontal gradients.  This is no
longer needed with the current approach.