Previously the cache would try to keep its size below a user-settable
value. This was not effective because when that value was too small,
it would cause a lot of churn every frame without actually keeping
the memory usage below the specified value.
We now evict buffer aggressively after they've not been used
(for two frames by default), but we don't cap the size of the cache.
The cache will naturally settle at the size it needs. When dynamic
resolution is used, it might be needed to increase resources
maximum age, which is a user-settable value still.
This improves performance on mobile on many scenes because the 64MB
default value was too low, causing the crash to thrash.
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