Simplifying stock palette indexing
Simplifying stock palette indexing

An attempt to avoid an integer overflow for 31 bit color depth.

Before:

```
Encoding          kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
----------------------------------------------------------------------------------------------------------------
jxl:m               18628 20623382    8.8568318   0.426   3.572   0.00004654   0.00000021  0.000001818792      0
jxl:d0.7            18628  4455150    1.9132902   2.301  14.336   1.59264278   0.47658482  0.911845053323      0
jxl:m:q95           18628  6402674    2.7496657   1.106   4.778   1.14431751   0.35536362  0.977131162284      0
jxl:m:p7:lp:P7      18628  7127590    3.0609851   0.084   6.182   6.79819155   0.61806553  1.891889392155      0
jxl:m:p0:lp:P0      18628 11385144    4.8894166   0.087   6.991   6.10237932   0.62416251  3.051790528555      0
Aggregate:          18628  8625275    3.7041749   0.380   6.380   0.32306191   0.02663974  0.098678258206      0
```

After:

```
Encoding          kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
----------------------------------------------------------------------------------------------------------------
jxl:m               18628 20623382    8.8568318   0.426   3.571   0.00004654   0.00000021  0.000001818792      0
jxl:d0.7            18628  4455150    1.9132902   2.309  14.259   1.59264278   0.47658482  0.911845053323      0
jxl:m:q95           18628  6402674    2.7496657   1.114   4.691   1.14431751   0.35536362  0.977131162284      0
jxl:m:p7:lp:P7      18628  7127590    3.0609851   0.080   6.420   6.79819155   0.61806553  1.891889392155      0
jxl:m:p0:lp:P0      18628 11385144    4.8894166   0.082   7.142   6.10237932   0.62416251  3.051790528555      0
Aggregate:          18628  8625275    3.7041749   0.373   6.426   0.32306191   0.02663974  0.098678258206      0
```

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