* fix(blend): Soft Light uses the W3C curve over dark backdrops (#8804)
Photoshop's Soft Light moves the backdrop towards D(cb) in the upper half,
and D(cb) is the cubic ((16·cb − 12)·cb + 4)·cb below a quarter, not
sqrt(cb). With sqrt the Soft Light cell of psd-tools rgb-blend-modes.psd
was up to 20 levels too light over the dark rainbow; with the cubic it is
within 4 of Photoshop's merged image.
Same change in compose.wgsl so the GPU compositor stays at CPU parity.
Krita's own soft_light (the sqrt curve) now opens as the approximation and
svg:soft-light / krita:soft_light_svg as the exact match, without warning.
Fixes#8804
* compose: the see-through Soft Light test expects the W3C curve over a dark backdrop (#8804)
The #6232 test computed Photoshop's Soft Light of yellow as sqrt(b) for red and
green. Its backdrop #04170F is below a quarter in both channels, where the W3C
curve (Photoshop's, measured in #8804) uses ((16b - 12)b + 4)b instead.
Gert Pellin - GPE
10a20743ea