The term `PIL` stands for "platform independent library." It exists since the `Initial Revision`
commit from 2002. Nowadays, we generally just use the `BLI` (blenlib) prefix for such code
and the `PIL` prefix feels more confusing then useful. Therefore, this patch renames the
`PIL` to `BLI`.
Pull Request: https://projects.blender.org/blender/blender/pulls/117325