Improve animation playback performance in EEVEE for materials using Mix
nodes. Socket availability was being set and reset on every evaluation
of Mix nodes, during animation playback, this was causing the graph to
be marked dirty, and the whole graph being re-evaluated on every frame,
causing performance issues during playback.
Additionally, do a bit of cleanup, traversing the node sockets with
the next link to improve clarity and reduce errors. Also refactoring
`nodeSetSocketAvailability` to early out and increase clarity on no-op.
Differential Revision: https://developer.blender.org/D16929
a3a60e9647