Nodes: Add support to mute node wires

This patch adds the ability to mute individual wires in the node editor.
This is invoked like the cut links operator but with a new shortcut.

Mute = Ctrl + Alt
Cut = Ctrl

Dragging over wires will toggle the mute state for that wire.
The muted wires are drawn in red with a bar across the center.
Red is used in the nodes context to indicate invalid links, muted links and internal links.

When a wire is muted it exposes the original node buttons which are normally hidden when a wire is connected.

Downstream and upstream links connected using reroute nodes are also muted.

Outside scope of patch:
- Add support for pynodes e.g. Animation Nodes
- Requires minor change to check for muted links using the `is_muted` link property or the `is_linked` socket property.

Maniphest Tasks: T52659

Differential Revision: https://developer.blender.org/D2807

2 files changed, 6 insertions, 2 deletions

diff --git a/source/blender/nodes/intern/node_exec.c b/source/blender/nodes/intern/node_exec.c
index 6207a1bf024..dd9d0b6796a 100644
--- a/source/blender/nodes/intern/node_exec.c
+++ b/source/blender/nodes/intern/node_exec.c
@@ -71,7 +71,8 @@ void node_get_stack(bNode *node, bNodeStack *stack, bNodeStack **in, bNodeStack
 static void node_init_input_index(bNodeSocket *sock, int *index)
 {
   /* Only consider existing link if from socket is valid! */
-  if (sock->link && sock->link->fromsock && sock->link->fromsock->stack_index >= 0) {
+  if (sock->link && !(sock->link->flag & NODE_LINK_MUTED) && sock->link->fromsock &&
+      sock->link->fromsock->stack_index >= 0) {
     sock->stack_index = sock->link->fromsock->stack_index;
   }
   else {
@@ -131,7 +132,7 @@ static struct bNodeStack *setup_stack(bNodeStack *stack,
   }
 
   /* don't mess with remote socket stacks, these are initialized by other nodes! */
-  if (sock->link) {
+  if (sock->link && !(sock->link->flag & NODE_LINK_MUTED)) {
     return ns;
   }
 
diff --git a/source/blender/nodes/intern/node_tree_ref.cc b/source/blender/nodes/intern/node_tree_ref.cc
index a66b7b1d2fe..8d979ffac9c 100644
--- a/source/blender/nodes/intern/node_tree_ref.cc
+++ b/source/blender/nodes/intern/node_tree_ref.cc
@@ -77,6 +77,9 @@ NodeTreeRef::NodeTreeRef(bNodeTree *btree) : btree_(btree)
   }
 
   LISTBASE_FOREACH (bNodeLink *, blink, &btree->links) {
+    if (blink->flag & NODE_LINK_MUTED) {
+      continue;
+    }
     OutputSocketRef &from_socket = this->find_output_socket(
         node_mapping, blink->fromnode, blink->fromsock);
     InputSocketRef &to_socket = this->find_input_socket(