DVB subtitles: Try to make some sense out of the PTS parsing code.

Also fix a typo which had no effect on the execution and remove member variables that were used only in one function.

Fixes CID 1233585.
Subtitle pin: Remove a redundant check.

pData can't be null by construction and checking the size is enough anyway.

Fixes CID 1233586.
PGS Subtitles: Ignore empty segments again now that the subtitle queue is fixed.

e664bace0fb7d5feecd0ed460c6b49f80916bd57 added the needed framework on the subtitle queue side.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...