Fix: Clicking on the some parts of the volume slider had no effect.

When clicking on the extreme start or end of the slider, the cursor moved but the volume wasn't changed.

This commit fixes #4684.
Updated LAV Filters to 0.62-13-04b5a00 (custom build based on 0.62-8-f581145).

The two custom patches used in d8a4b619d25ff73d36817f39630c2ccea61be357 have been upstreamed.
Comment out some unused variable assignments.

Those can be useful if the functions are extended one day.
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...