Prior to this commit, variables inside `StrokeCache` used either a
`true_` prefix to indicate that the relevant position / normal /
displacement was not modified by symmetry, or a `_symmetry` / `_symm`
suffix to indicate that it was.
This commit changes all such variables to have a `_symm` suffix, as
that has fewer connotations than the `true_` prefix.
Pull Request: https://projects.blender.org/blender/blender/pulls/126902
b019e4235a