Part of overall "improve filtering situation" (#116980): replace Subsampled3x3
(added for blender 3.5 in f210842a72 et al.) strip scaling filter with a
general Box filter.
Subsampled3x3 is really a Box filter ("average pixel values over NxM region"),
hardcoded to 3x3 size. As such, it works pretty well when downscaling images by
3x on each axis. But starts to break down and introduce aliasing at other
scaling factors. Also when scaling up or scaling down by less than 3x, using
total of 9 samples is a bit of overkill and hurts performance.
So instead, calculate the amount of NxM samples needed by looking at scaling
factors on X/Y axes. Note: use at least 2 samples on each axis, so that when
rotation is present, the result edges will get some anti-aliasing, just like it
was happening in previous filter implementation.
Images in PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/117584
5bd1e0bb22