CD-i: Refactoring and Renaming

This intentionally makes no changes to output. The purpose is to make the code easier to understand and maintain.

1. The non-canonical name "Regions" was changed to the official name "Mattes". Corresponding abbreviation "R" is changed to "M".
2. Deprecated function get_plane_width is deleted.
3. mix_lines is refactored.
4. Minor changes.

Author: Vincent-Halver