Improve the correctness of the code handling EC_BUFFERING_DATA events.

There is no reason to treat the event parameter as HRESULT. Also ensure the buffering state is always reset when closing a file.

Although the code was somewhat wrong, this commit should be cosmetic only in practice.
LAV Filters: Distribute CHANGELOG.txt, README.txt, COPYING files (PoC).
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.
WebUI: Make use of new [hasaudio] info.