Generalising the 16x16 and 32x32 square splitting (#288)
Generalising the 16x16 and 32x32 square splitting (#288)

Removes code, prepares codebase for 64x64 square splitting.
Before:

```
Compr                 Input    Compr            Compr       Compr  Decomp  Butteraugli
Method               Pixels     Size              BPP   #    MP/s    MP/s     Distance    Error p norm   PSNR   QABPP     DCT2     DCT4   DCT4X8      AFV     DCT8  DCT8X16  DCT8X32    DCT16 DCT16X32    DCT32           BPP*pnorm   Errors
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
jxl:d0.8:epf0      17972865  3978806    1.77102804700   1   2.366  14.363   1.57902789   0.52086395504  41.78   2.105 0.000000 0.000911 0.229583 0.021543 0.060425 0.226524 0.000000 0.378839 0.019399 0.062045  0.9224646730469741        0
jxl:d1:epf1        17972865  3463827    1.54180293459   1   2.582  19.872   1.94598019   0.60485873749  40.68   2.119 0.000000 0.000911 0.205023 0.021429 0.055422 0.201434 0.000000 0.368441 0.040452 0.106485  0.9325729764825031        0
jxl:d2:epf3        17972865  2237796    0.99607758696   1   2.794  16.521   3.58487940   0.95352175925  37.47   2.246 0.000000 0.000911 0.150402 0.021098 0.043984 0.157941 0.000000 0.264263 0.163603 0.198272  0.9497816530707661        0
jxl:d3:epf0        17972865  1669569    0.74315096675   1   2.760  26.112   4.53973532   1.27958860382  35.22   2.301 0.000000 0.001139 0.121537 0.019339 0.038903 0.141895 0.000000 0.199824 0.238297 0.241117  0.9509275079664137        0
jxl:d4:epf3        17972865  1382640    0.61543443408   1   2.433  15.144   7.13622332   1.56464959734  34.06   2.422 0.000000 0.001139 0.098897 0.017430 0.032984 0.129639 0.000000 0.167320 0.287295 0.268465  0.9629392394641252        0
jxl:d8:epf1        17972865   809952    0.36052215381   1   2.786  24.425   9.39446449   2.50509222536  30.89   2.362 0.000000 0.001367 0.055906 0.011583 0.019691 0.101479 0.000000 0.114476 0.371247 0.329940  0.9031412445869741        0
jxl:d16:epf3       17972865   474716    0.21130342881   1   2.791  14.753  19.64636612   4.43095030955  28.64   2.691 0.000000 0.001367 0.019278 0.004486 0.005380 0.061888 0.000000 0.068569 0.387428 0.458931  0.9362749932844370        0
jxl:d24:epf3       17972865   360283    0.16036753183   1   2.597  14.258  21.55065346   5.77995514017  27.36   2.672 0.000000 0.001367 0.010209 0.002296 0.002766 0.045522 0.000000 0.050821 0.364467 0.530378  0.9269171399281144        0
jxl:d32:epf3       17972865   293127    0.13047535827   1   2.896  14.217  25.75268745   7.00993165520  26.54   2.671 0.000000 0.001367 0.005708 0.001338 0.001659 0.035231 0.000000 0.039725 0.335211 0.587808  0.9146233441551781        0
Aggregate:         17972865  1125191    0.50083983297 ---   2.662  17.264   6.92351368   1.86311414973  33.21   2.389 0.000000 0.001147 0.055917 0.009308 0.016526 0.103265 0.000000 0.139138 0.175065 0.251342  0.9331217795490244        0
```

After:

```
Compr                 Input    Compr            Compr       Compr  Decomp  Butteraugli
Method               Pixels     Size              BPP   #    MP/s    MP/s     Distance    Error p norm   PSNR   QABPP     DCT2     DCT4   DCT4X8      AFV     DCT8  DCT8X16  DCT8X32    DCT16 DCT16X32    DCT32           BPP*pnorm   Errors
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
jxl:d0.8:epf0      17972865  3979274    1.77123636104   1   2.356  14.049   1.57907867   0.52082615981  41.78   2.105 0.000000 0.000911 0.229903 0.021550 0.060596 0.225890 0.000000 0.378967 0.019399 0.062045  0.9225062320418125        0
jxl:d1:epf1        17972865  3464334    1.54202860813   1   2.543  19.901   1.94598019   0.60485140445  40.68   2.120 0.000000 0.000911 0.205280 0.021422 0.055578 0.200900 0.000000 0.368512 0.040509 0.106485  0.9326981693215983        0
jxl:d2:epf3        17972865  2238156    0.99623782853   1   2.868  16.411   3.58487940   0.95343768671  37.47   2.243 0.000000 0.000911 0.150687 0.021091 0.044091 0.157471 0.000000 0.264263 0.163688 0.198272  0.9498506906411841        0
jxl:d3:epf0        17972865  1669650    0.74318702110   1   2.764  26.253   4.53973532   1.27948292271  35.22   2.300 0.000000 0.001139 0.121659 0.019385 0.039020 0.141582 0.000000 0.199796 0.238354 0.241117  0.9508951018782891        0
jxl:d4:epf3        17972865  1382386    0.61532137475   1   2.418  14.684   7.13622332   1.56481193158  34.06   2.424 0.000000 0.001139 0.099000 0.017466 0.033041 0.129425 0.000000 0.167277 0.287352 0.268465  0.9628622289681604        0
jxl:d8:epf1        17972865   810445    0.36074159573   1   2.794  23.381   9.39446449   2.50537426898  30.89   2.364 0.000000 0.001367 0.055959 0.011601 0.019756 0.101400 0.000000 0.114362 0.371304 0.329940  0.9037927117020400        0
jxl:d16:epf3       17972865   474664    0.21128028280   1   2.836  14.689  19.64636612   4.43111770848  28.64   2.690 0.000000 0.001367 0.019286 0.004486 0.005409 0.061896 0.000000 0.068384 0.387570 0.458931  0.9362078025868050        0
jxl:d24:epf3       17972865   360266    0.16035996487   1   2.556  13.278  21.55065346   5.77990439914  27.36   2.672 0.000000 0.001367 0.010219 0.002296 0.002777 0.045551 0.000000 0.050707 0.364524 0.530378  0.9268652663933534        0
jxl:d32:epf3       17972865   293086    0.13045710854   1   2.785  13.879  25.75268745   7.01006938900  26.54   2.670 0.000000 0.001367 0.005726 0.001342 0.001655 0.035210 0.000000 0.039668 0.335268 0.587808  0.9145133831221164        0
Aggregate:         17972865  1125266    0.50087330725 ---   2.652  16.892   6.92353842   1.86311612031  33.21   2.389 0.000000 0.001147 0.055992 0.009317 0.016568 0.103117 0.000000 0.139026 0.175128 0.251342  0.9331851329655315        0
```

Looked very close the same in manual viewing.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl