Improve/unify approach to line indexing.

* Use consistently type MD_SIZE for line indeces.
* Remove pointer arithmetic if lines and replace it with line index
  arithmetic.

This resolves some warnings in MSVC builds.
See PR #232.

Co-authored-by: Martin Mitas <mity@morous.org>
Co-authored-by: Shawn Rutledge <s@ecloud.org>