Improve the entity path filter mutation API

[abey79]

Currently, all methods mutating the entity path filter in a ViewContents write the mutated filter to the blueprint store but leaves the deserialised version unchanged. This means that all mutations are lost except the last one within a frame, which is a major foot-gun. We should fix this API such as to allow arbitrary number of mutations during a frame (while retaining interior mutability for the ViewportBlueprint structure and friends).