Text subtitles: Opaque boxes were scaled twice.

Fixes #4991.
Text subtitles: "opaque box" outlines will now be drawn even if the border width is set to 0.

The size of the text is independent of the border width so there is no reason not to draw that part.

Ticket #4991.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.