This commit refactors the ImageCacheSupplier to act as a
proper cache. It is used by the ImageExtesion and all operations
on the images are propagated to the cache.
A method has also been added to the TemplateBuilder to let users
force a cache refresh. There have been several requests to provide a way
to disable image caching in the compute abstraction, and this new method
should fix that.
40f31786c5