PNG out-of-range palette indices

I haven't looked too much into it but my initial tests with [spngt](https://github.com/libspng/spngt) on the two images at https://github.com/w3c/PNG-spec/issues/96 appear to not decode with wuffs, probably because the out-of-range values are handled as an error, might want to change it to match libpng's defaults.