Adjustments to the cache: We remove images from the cache only if the
hash is different. This is the best condition I could find that would
suit the following use case:

- The user tweaks a parameter on the user interface
- The user is playing a sequence
- The user is seeking the timeline

This version makes the NodeCache take a little more space but is more
effective overall because we don't wipe images on time change, and
these images could still be accessed anyway.