Some RNA properties and operators did not invalidate cache or did it
incorrectly.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D13101
This value was meant to be used for keeping images that are slowest to
render in cache. Method of measurement was flawed, because it doesn't
take UI overhead into consideration.
Cache panel is to be removed because users should not have to tweak
settings like this. It is not useful for development either, therefore
it is removed completely.