Automatically hide unused (=unlinked) node sockets in collapsed ("hidden") nodes...

Automatically hide unused (=unlinked) node sockets in collapsed ("hidden") nodes. This is to make nodes with many sockets cleaner and simpler, which is the purpose of collapsing nodes in the first place.

The hiding code uses the SOCK_IN_USE flags already present. These were only temporarily set by the shader node code for determining needed texture coordinate types. Now they are used persistently and updated along with the sock->link pointers.