This affected cursor snapping as well as the `Jump to Keyframes`
operator since both use `sum_selected_keyframes` to get the averaged
values of elements. It was already taking into account the normalization
factor, but the way a single `KeyframeEditData` was created from
multiple `bAnimListElem` was wrong:
- first, we should only add to it if we actually have some keys selected
(not a problem without normalization, but with it, we would add values
there
- second, the offset needs to be taken into account for each selected
key when adding to the absolute values
Could go into LTS probably
Pull Request: https://projects.blender.org/blender/blender/pulls/125221
163baad17e