Minor speedup to ac strategy for special case 8x8 (#208)
Minor speedup to ac strategy for special case 8x8 (#208)

No quality change, possibly 1 % speedup at default speed, more at higher
speeds if the new ac strategy heuristic was used there.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl