This commit improves the drawing of selected node links:
- Highlight the entire link to make it easier to spot where the link
is going/coming from.
- Always draw selected links on top, so they are always clearly
visible.
- Don't fade selected node links when the sockets they are connected
to are out out view.
- Dragged node links still get a partial highlight when they are only
attached to one socket.
Differential Revision: https://developer.blender.org/D11930
75bb99fa40