vtkImprintFilter: use `int8_t` for `{Cell,Point}Classification`

`char` on aarch64 is `unsigned` and the negative variants for
`PointClassification` end up leaking out when compared with literals
where promotion occurs (as `int`). While `CellClassification` lacks
negative variants, using its enum name helps to keep usage consistent.

Assign the `int8_t` storage size for these `enum` types and store its
type explicitly where used.