Add u8 fast path for some blend modes

[LaurenzV]

Mostly generated with codex, but I did look at it myself and make some adjustments, so I hope it's good now. Since we do have quite a few tests for blending (both manual ones as well as via COLR), not too concerned about correctness issues here.

Note that this does not address #1579 yet so it's possible this won't have much effect on AVX2. However, on NEON I'm seeing 4x-5x speedups for blending now:

fine/blend/normal_u8_neon
                        time:   [40.096 ns 40.297 ns 40.521 ns]
                        change: [-1.1748% +0.8265% +3.1392%] (p = 0.45 > 0.05)
                        No change in performance detected.
Found 8 outliers among 100 measurements (8.00%)
  3 (3.00%) high mild
  5 (5.00%) high severe

fine/blend/multiply_u8_neon
                        time:   [976.39 ns 978.78 ns 981.34 ns]
                        change: [-80.790% -80.728% -80.664%] (p = 0.00 < 0.05)
                        Performance has improved.

fine/blend/screen_u8_neon
                        time:   [1.0140 µs 1.0167 µs 1.0199 µs]
                        change: [-80.496% -80.407% -80.320%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

fine/blend/overlay_u8_neon
                        time:   [1.3631 µs 1.3667 µs 1.3701 µs]
                        change: [-74.682% -74.592% -74.499%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

fine/blend/darken_u8_neon
                        time:   [1.1359 µs 1.1385 µs 1.1412 µs]
                        change: [-77.273% -77.197% -77.125%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild

fine/blend/lighten_u8_neon
                        time:   [1.1535 µs 1.1557 µs 1.1582 µs]
                        change: [-77.013% -76.936% -76.857%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

fine/blend/color_dodge_u8_neon
                        time:   [5.6951 µs 5.7070 µs 5.7195 µs]
                        change: [+1.6232% +1.9789% +2.3529%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe

fine/blend/color_burn_u8_neon
                        time:   [5.6208 µs 5.6334 µs 5.6466 µs]
                        change: [+1.5668% +1.9000% +2.2646%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

fine/blend/hard_light_u8_neon
                        time:   [1.3581 µs 1.3602 µs 1.3626 µs]
                        change: [-75.426% -75.345% -75.265%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe

fine/blend/soft_light_u8_neon
                        time:   [6.0497 µs 6.0630 µs 6.0768 µs]
                        change: [+1.2844% +1.7849% +2.2700%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 5 outliers among 100 measurements (5.00%)
  3 (3.00%) high mild
  2 (2.00%) high severe

fine/blend/difference_u8_neon
                        time:   [1.2694 µs 1.2720 µs 1.2747 µs]
                        change: [-75.605% -75.514% -75.423%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe

fine/blend/exclusion_u8_neon
                        time:   [1.0596 µs 1.0614 µs 1.0634 µs]
                        change: [-80.316% -80.250% -80.184%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  2 (2.00%) high mild
  3 (3.00%) high severe

fine/blend/hue_u8_neon  time:   [8.5128 µs 8.5387 µs 8.5659 µs]
                        change: [+1.5041% +1.9534% +2.4143%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 5 outliers among 100 measurements (5.00%)
  5 (5.00%) high mild

fine/blend/saturation_u8_neon
                        time:   [8.5693 µs 8.6052 µs 8.6431 µs]
                        change: [+1.7844% +2.2460% +2.6872%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

fine/blend/color_u8_neon
                        time:   [7.5338 µs 7.5591 µs 7.5869 µs]
                        change: [+0.7948% +1.2293% +1.6653%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild

fine/blend/luminosity_u8_neon
                        time:   [7.5325 µs 7.5531 µs 7.5772 µs]
                        change: [+0.8659% +1.3864% +1.8684%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 6 outliers among 100 measurements (6.00%)
  3 (3.00%) high mild
  3 (3.00%) high severe