The SATURATED_LAYOUT_ARITHMETIC flag is intended to protect against potential arithmetic overflow certain layout and rendering operations once the SUBPIXEL_LAYOUT is enabled. After some discussion in the mailing list, it seems that enabling SATURATED_LAYOUT_ARITHMETIC whenever the SUBPIXEL_LAYOUT flag is enabled is more than recommended. Since the SATURATED_LAYOUT_ARITHMETIC flag makes no sense without SUBPIXEL_LAYOUT, actually there is an assertion preventing this, it seems that the most sensible action is to merge the SATURATED_LAYOUT_ARITHMETIC logic into the SUBPIXEL_LAYOUT, and eventually removing it.
fixed by bug 136106