Multiple performance regressions in 3.3.1 compared to 3.2.1

There appear to be multiple performance regressions in OpenEXR 3.3.1 when compared to OpenEXR 3.2.1. These regressions were first observed in real-world use in an application. To facilitate regression testing, I have backported `exrmetrics` to build against OpenEXR 3.2.1 and created a simple series of tests. Please see https://github.com/peterurbanec/exr_perf

To summarise, largest real-world performance losses are observed with DWA compression, where the slow down is up to 58% when writing half images. Results from exrmetrics also show slowdowns with DWA, although at only 15%.

More details, taken from https://github.com/peterurbanec/exr_perf/README.md

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Change in performance, measured with backported version of exrmetrics from this repository
Quantity measured is time taken for one frame, therefore positive Δ % quantities indicate
a performance regression. These figures were collected on AMD Ryzen Threadripper PRO 3975WX
running Linux

```
exrmetrics 3.2.1 3.3.1 3.2.1 3.3.1
Encode time Encode time Δ % Decode time Decode time Δ %
none-half 0.0506104 0.0538534 6% 0.0147626 0.00607923 -59%
none-float 0.1009070 0.1006210 -0% 0.0210363 0.00938430 -55%
rle-half 0.0742712 0.0735408 -1% 0.0315694 0.02286890 -28%
rle-float 0.1433720 0.1428730 -0% 0.0468555 0.03499330 -25%
zips-half 0.1403540 0.1435510 2% 0.0413822 0.03292240 -20%
zips-float 0.2781060 0.2841830 2% 0.0759776 0.06599230 -13%
zip-half 0.1490650 0.1478110 -1% 0.0362129 0.02737780 -24%
zip-float 0.3176970 0.3146880 -1% 0.0757593 0.06454440 -15%
piz-half 0.1664580 0.1329990 -20% 0.0470330 0.04469030 -5%
piz-float 0.5811040 0.5301250 -9% 0.1553520 0.11521600 -26%
pxr24-half 0.1577990 0.1601900 2% 0.0300827 0.02612540 -13%
pxr24-float 0.2518220 0.2486510 -1% 0.0582780 0.03983260 -32%
b44-half 0.0790941 0.0811910 3% 0.0132777 0.01222920 -8%
b44-float 0.1036020 0.1121230 8% 0.0210958 0.00707329 -67%
b44a-half 0.0723179 0.0742420 3% 0.0113973 0.01050320 -8%
b44a-float 0.1024170 0.1032410 0% 0.0214426 0.00713971 -67%
dwaa-half 0.2754550 0.2919960 6% 0.0386784 0.04521370 17% *
dwaa-float 0.3004660 0.3176720 6% 0.0591789 0.06652650 12% *
dwab-half 0.2452280 0.2807360 15% 0.0395200 0.04543520 15% *
dwab-float 0.2713410 0.2937500 8% 0.0637925 0.06695870 5% *

```

Change in performance as observed in a real-world application. Quantity measured is frames
per second, therefore a negative Δ % is a performance regression. These figures were collected
on i7-13800H running Windows.

```
3.2.1 3.3.1 3.2.1 3.3.1
Encode fps Encode fps Δ % Decode fps Decode fps Δ %
HD-half-DWA32(45) 28.2 11.8 -58 71.8 66.2 -8 *
HD-float-DWA32(45) 16.7 9.0 -46 31.0 35.9 15 *
HD-half-DWA256(45) 16.4 14.8 -10 43.1 38.7 -10 *
HD-float-DWA256(45) 9.4 9.2 -2 21.7 24.5 -13 *
HD-half-piz 31.7 29.7 -6 64.4 58.6 -9 *
HD-half-Pxr24 19.2 18.9 -2 79.5 73.6 -7
HD-float-Pxr24 20.4 24.2 19 39.6 43.2 9
HD-half-B44A 19.3 19.8 3 70.8 68.2 -3
HD-float-B44A 17.7 18.1 2 32.4 40.7 26

```

Rows marked with * are of particular concern.

Start with the backported exrmetrics tests and the linked exr_perf README, reproducing the 3.2.1 versus 3.3.1 measurements on the reported compression modes. Trace the largest DWA encoding regressions into the relevant OpenEXR implementation, then verify that benchmark results improve without breaking other codecs.

Written by the indexing model from the issue text.