tweak size header encoding (512x256 can be 'small' too)
tweak size header encoding (512x256 can be 'small' too)

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl