[raster] Handle `FT_RASTER_FLAG_AA' correctly. This fixes a breakage caused by the commit `[raster] Remove 5-level gray AA mode from monochrome rasterizer.'. Problem reported by Markus Trippelsdorf <markus@trippelsdorf.de> and octoploid <octoploid@yandex.com>. * src/raster/ftraster.c (ft_black_render): Handle `FT_RASTER_FLAG_AA'. * src/raster/ftrend1.c (ft_raster1_render): Remove gray AA mode remnants.