Increase quantization in flat 8x8 blocks. (#1675)
Increase quantization in flat 8x8 blocks. (#1675)

Benchmark results with all 8x8 blocks:

Before
```
Encoding      kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
------------------------------------------------------------------------------------------------------------
jxl:d1          13270  2723911    1.6420912   1.810  16.222   1.74343252   0.65861527  1.081506359483      0
jxl:d2          13270  1706943    1.0290190   1.857  14.973   2.94722962   1.08186301  1.113257580031      0
jxl:d4          13270  1000052    0.6028746   1.782   9.602   5.04913187   1.77373730  1.069341081351      0
```

After:
```
Encoding      kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
------------------------------------------------------------------------------------------------------------
jxl:d1          13270  2755241    1.6609783   2.096  17.783   1.73951459   0.64075941  1.064287489864      0
jxl:d2          13270  1740389    1.0491817   2.147  16.969   2.95925331   1.04322046  1.094527792322      0
jxl:d4          13270  1047406    0.6314216   2.062  10.975   5.06505680   1.67949491  1.060469345796      0
```

Benchmark results with normal AC strategy:

Before
```
Encoding      kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
------------------------------------------------------------------------------------------------------------
jxl:d1          13270  2680680    1.6160297   2.319  17.152   1.68643689   0.59921594  0.968350771923      0
jxl:d2          13270  1683824    1.0150819   2.409  17.450   3.07402062   0.94836343  0.962666511883      0
jxl:d4          13270  1001875    0.6039735   2.295  10.250   5.01221323   1.50586670  0.909503632879      0
```

After
```
Encoding      kPixels    Bytes          BPP  E MP/s  D MP/s     Max norm        pnorm       BPP*pnorm   Bugs
------------------------------------------------------------------------------------------------------------
jxl:d1          13270  2682080    1.6168737   2.431  18.466   1.68643689   0.59894225  0.968413965890      0
jxl:d2          13270  1685344    1.0159982   2.575  18.422   3.07402062   0.94758179  0.962741367927      0
jxl:d4          13270  1004383    0.6054855   2.411  10.610   5.01221752   1.50244186  0.909706712628      0
```
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl