Improve scene video filter to write pictures in a separate thread.

macosx: trust the user that s/he knows what s/he's doing when opening a VIDEO_TS folder (close #6108)

in other words, accept folders which aren't named 'VIDEO_TS'

It probably wouldn't hurt to provide helpers for mouse events.

This is similar to hotkeys. There is no need for a hand-over thread.

Thanks to Rémi for the help

If a thread can be cancelled, it can be joined.

Regression introduced in 55c6f688.

There is no matching vlc_object_alive() or vlc_object_waitpipe().

vlc_cancel() terminates the thread. There is a massive leak though.

This reverts commit a11c642f.

This reverts commit 4dc0b22b.

The channels count can trivially be computed with popcount().

It should be called before exit can happen.
This restores reverted commit 4d636fa3.

In my opinion, amplifying the audio that gets recorded to a file may do
more harm than good. Indeed, there is no direct feedback to detect and
correct saturation or distortion.