Scene audio volume changes require the scene to be tagged with
`ID_RECALC_AUDIO_VOLUME` (see `BKE_scene_update_sound()`). Tagging
happens in the RNA update function `rna_Scene_volume_update()`, but that
function is not called by the animation system. As a result, animated
volume changes are not sent to the audio system.
This commit adds a new depsgraph operation node that sets this tag when
necessary, so that the animated values are used in the rest of the
depsgraph evaluation.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D7429
6adb254bb0