This makes it so the stroke selection is taken into account by the
interpolation tool.
This does NOT use the selection _order_, just limits interpolation by
index to the selection. The result will be the subset of selected
strokes, excluding non-selected strokes, which is closer to the GPv2
behavior.
The `sample_curve_attribute` function was using the target curve index
for both the src and dst curves. This worked when all the curves are
interpolated, but with selections the dst curve is generally not the
same index as the src curve. The `from_curve_indices` array must be
passed along as well to retrieve the correct source index.
Pull Request: https://projects.blender.org/blender/blender/pulls/129096
297b97f2df