Preparations for non-aligned 16x16 comparisons (#230)
Preparations for non-aligned 16x16 comparisons (#230)

Preparation work ac strategy improvements. This PR tries to be a no
functional changes.

This PR prepares the 16X16 ac strategy lookup so that it can
soon be used within non-aligned positions in the 64x64 block.

Before and after are the same:

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      17448577  3870714    1.77468409028   1   2.246  14.342   1.80222321   0.52939698283  41.95   2.125 0.000000 0.000000 0.188362 0.006371 0.062461 0.346435 0.000000 0.040317 0.194810 0.125765  0.9395124028640433        0
jxl:d1:epf1        17448577  3365010    1.54282380735   1   3.069  21.676   1.90873623   0.61329670189  40.79   2.179 0.000000 0.000000 0.170558 0.006576 0.055961 0.329885 0.000000 0.046274 0.219547 0.139146  0.9462087526504965        0
jxl:d2:epf3        17448577  2172907    0.99625637094   1   3.125  17.243   3.79021478   0.96497337332  37.43   2.252 0.000000 0.000234 0.124100 0.007002 0.036440 0.263664 0.000000 0.064364 0.293961 0.186330  0.9613608709484811        0
jxl:d3:epf0        17448577  1618162    0.74191127448   1   2.448  25.183   4.76765537   1.28786214751  35.19   2.314 0.000000 0.000234 0.092772 0.006327 0.027802 0.226993 0.000000 0.072038 0.336333 0.222716  0.9554794472159012        0
jxl:d4:epf3        17448577  1344831    0.61659171404   1   2.968  15.779   6.82338524   1.56208631217  34.06   2.425 0.000000 0.000234 0.073758 0.005285 0.019916 0.193042 0.000000 0.077011 0.371369 0.249066  0.9631694767031896        0
jxl:d8:epf1        17448577   796979    0.36540698992   1   3.006  24.874   9.87168598   2.45571728749  30.94   2.378 0.000000 0.000234 0.030861 0.002574 0.004775 0.110140 0.000000 0.079843 0.444170 0.327648  0.8973362621231278        0
jxl:d16:epf3       17448577   450579    0.20658601558   1   2.546  14.262  21.56148529   4.44012931344  28.56   2.553 0.000000 0.000234 0.006437 0.000436 0.000590 0.045658 0.000000 0.051585 0.421517 0.480233  0.9172686235303881        0
jxl:d24:epf3       17448577   339753    0.15577339058   1   3.126  14.443  21.47612762   5.78196502410  27.33   2.580 0.000000 0.002817 0.002461 0.000084 0.000212 0.028829 0.000000 0.036840 0.377209 0.559402  0.9006762959908683        0
jxl:d32:epf3       17448577   276995    0.12699946821   1   3.139  14.963  24.38039017   7.02440096331  26.53   2.600 0.000000 0.033568 0.001181 0.000066 0.000121 0.022044 0.000000 0.026643 0.324537 0.600013  0.8920951868252788        0
Aggregate:         17448577  1084859    0.49739733676 ---   2.833  17.615   7.12413578   1.86962978654  33.21   2.373 0.000000 0.000680 0.029267 0.001580 0.005565 0.117348 0.000000 0.051817 0.320809 0.276844  0.9299488765607850        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      17448577  3870714    1.77468409028   1   2.223  14.077   1.80222321   0.52939698283  41.95   2.125 0.000000 0.000000 0.188362 0.006371 0.062461 0.346435 0.000000 0.040317 0.194810 0.125765  0.9395124028640433        0
jxl:d1:epf1        17448577  3365010    1.54282380735   1   3.086  21.892   1.90873623   0.61329670189  40.79   2.179 0.000000 0.000000 0.170558 0.006576 0.055961 0.329885 0.000000 0.046274 0.219547 0.139146  0.9462087526504965        0
jxl:d2:epf3        17448577  2172907    0.99625637094   1   3.141  17.084   3.79021478   0.96497337332  37.43   2.252 0.000000 0.000234 0.124100 0.007002 0.036440 0.263664 0.000000 0.064364 0.293961 0.186330  0.9613608709484811        0
jxl:d3:epf0        17448577  1618162    0.74191127448   1   2.416  24.477   4.76765537   1.28786214751  35.19   2.314 0.000000 0.000234 0.092772 0.006327 0.027802 0.226993 0.000000 0.072038 0.336333 0.222716  0.9554794472159012        0
jxl:d4:epf3        17448577  1344831    0.61659171404   1   3.017  15.704   6.82338524   1.56208631217  34.06   2.425 0.000000 0.000234 0.073758 0.005285 0.019916 0.193042 0.000000 0.077011 0.371369 0.249066  0.9631694767031896        0
jxl:d8:epf1        17448577   796979    0.36540698992   1   3.041  24.387   9.87168598   2.45571728749  30.94   2.378 0.000000 0.000234 0.030861 0.002574 0.004775 0.110140 0.000000 0.079843 0.444170 0.327648  0.8973362621231278        0
jxl:d16:epf3       17448577   450579    0.20658601558   1   2.454  13.966  21.56148529   4.44012931344  28.56   2.553 0.000000 0.000234 0.006437 0.000436 0.000590 0.045658 0.000000 0.051585 0.421517 0.480233  0.9172686235303881        0
jxl:d24:epf3       17448577   339753    0.15577339058   1   3.120  14.824  21.47612762   5.78196502410  27.33   2.580 0.000000 0.002817 0.002461 0.000084 0.000212 0.028829 0.000000 0.036840 0.377209 0.559402  0.9006762959908683        0
jxl:d32:epf3       17448577   276995    0.12699946821   1   3.165  14.707  24.38039017   7.02440096331  26.53   2.600 0.000000 0.033568 0.001181 0.000066 0.000121 0.022044 0.000000 0.026643 0.324537 0.600013  0.8920951868252788        0
Aggregate:         17448577  1084859    0.49739733676 ---   2.828  17.453   7.12413578   1.86962978654  33.21   2.373 0.000000 0.000680 0.029267 0.001580 0.005565 0.117348 0.000000 0.051817 0.320809 0.276844  0.9299488765607850        0
```
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl