Add an option and calculate Y-offset depending on it (from top or bottom).

Why this needed.

For instance if subtitle line is multiline and the subtitles are supposed to be located on top of screen the relative offset will be calculated from the baseline which is bottom. This leads to the following situation when you offset subtitles to the top of screen there is a risk some part of subtitles will be overlaped. If baseline would be top this would not happen I guess. To be more clear take a look on a screenshot