The interpolate tool was using a linear re-sampling of the curves, which changes
non-uniform point distributions can causes unexpected shifts in points along the
curve, even very close to an input stroke (mix factor ~0 or ~1).
This patch adds an alternative interpolation function with explicit segment
indices and fractions, which can then be computed by the interpolation tool.
The point segment factors are chosen such that each point of the input strokes
has an exact matching point in the interpolation. When the factor is close to
0 or 1 the shape of the curve thus matches the respective input exactly.
This approach is more similar to what GPv2 did, except instead of sub-dividing
each segment it simply generates a _local_ uniform sample to fit more points
into the same segment. These extra points are colinear at the extremes of the
mix factor range, so the curve matches the input curves visually.
Pull Request: https://projects.blender.org/blender/blender/pulls/130594
bca8fc76f9