Fix line algorithm in raster engine to match current rasterization rules

In 4.8 we changed the raster engine's handling of subpixel positioning to be more in line with floating point drawing systems, like GL and VG. The cosmetic line algorithm, drawLine_midpoint_i does not support this mode so we currently have some non-straight edges where outlines are not correctly aligned to the polygon edge. The outlineFillConsistency autotest in qpainter is currently disabled as a result of it.