Only compute store/caching stats when the memory panel is opened

[teh-cmc]

Computing stats for the store and caches is a very costly process.

We do so every frame because the memory panel's data gets updated whether it is opened or not, so that the memory graphs are not missing any data when you eventually open it.

In the case of the store, we've added a lot of complexity a long time ago to make that process fast enough (although in a big enough scene, it will still take a non-trivial amount of time).
I've recently gone through the process of adding the same kind of complexity to the caches, but it gets absolutely insane -- so insane that it's just not worth it.

So, instead of making the caches even more complicated than they already are just so the memory panel can get updated in the background, we've decided to just not updated the mempanel stats when it's hidden.

Before:

After (mempanel opened):

After (mempanel hidden):

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[Wumpf]

adding it to the changelog since it's such a big perf impact on some scenes

[Wumpf]

so simple <3