Subtitles: Improve error checking with ASS rendering.

Some crashes could occur when the SSA script was using obviously wrong values.

Use the same check than libass for maximum glyph size.
Replace "On top > Never" by "On top > Default".

In most of the case "On top > Default" will have the exact same behavior than "On top > Never" but it won't try to override the "on top" flag if an external tool sets it.

This commit fixes #2689.