Embedded VobSub track: honor the per-subtitle forced flag when the index tells to do so.
External VobSub track: Fix the per-subtitle forced flag.
This flag was currently never honored because the track setting was mistakenly overwritten by the Subresync bar.
PGS subtitles: Prepare support for the per-subtitle forced flag.

Currently this feature is always turned off and can't be controlled by the user.
First step for per-subtitle forced flag control.
SubPicImpl: Check that the static resource is big enough.
PGS Subtitles: Improve rendering.

Store and render only the bounding box containing the subtitles instead of the full frame.
This should fix not appearing or wrongly resized subtitles and improve slightly the rendering performance.

This commit fixes ticket #1838.