[BUG] Frame skips caused by forced subtitles

Hello!

I originally reported this bug in the LibreELEC AMLogic forum:
https://forum.libreelec.tv/thread/8100-b…subtitles/

However, it turns out this is a global Krypton bug, since I was able to reproduce it on a RPi2 and others reproduced it in Windows.

Issue description:
Certain types of forced subtitles from a Blu-ray cause one or two frame skips every time a new line of subtitle is displayed.

This does not happen in Jarvis builds.
It also does not happen in my Leia nightly build in Windows.

How to reproduce:
Play linked sample file from Avatar (Blu-ray remux) and monitor PlayerDebug for frame skips.

The subtitle in question seems to be a special (rare) kind, since it looks like it is part of the global English subtitle, which does not have a forced flag.
However, it is still being displayed automatically for foreign language parts.
The font is different then, too (quite exotic).

Sample:
https://ufile.io/u9u48

Log:
https://ufile.io/r2v42