snd_mutewhenidle: add only-when-minimised mode, fix cl_capturevideo compat, cleanup

snd_mutewhenidle: add only-when-minimised mode, fix cl_capturevideo compat, cleanup

Maybe snd_mutewhenidle 2 should be the default?

When inactive/minimised with snd_mutewhenidle enabled, the audio capture
was unreliable and sound was sometimes output when it should have been
muted.
Closes https://gitlab.com/xonotic/darkplaces/-/issues/92
(the video issue was already fixed).

Timedemos now share the same mute implementation as snd_mutewhenidle.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>