Fix T75686: Animating scene audio volume doesn't work

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

source/blender/depsgraph/intern/node/deg_node_operation.cc

@@ -61,6 +61,8 @@ const char *operationCodeAsString(OperationCode opcode)
     /* Scene related. */
     case OperationCode::SCENE_EVAL:
       return "SCENE_EVAL";
+    case OperationCode::AUDIO_VOLUME:
+      return "AUDIO_VOLUME";
     /* Object related. */
     case OperationCode::OBJECT_BASE_FLAGS:
       return "OBJECT_BASE_FLAGS";