Fix LoD being activated for everything, even for brushes that do not support it

Well, my refactoring was a bit too optimistic :) A special read-only
resource (EffectiveLodAvailablility) was actually needed for brushes
to distinguish the case when user allows LoD, but the brush doesn't
support it (and it is known only by the GUI, not by the brush itself).

BUG:391050
(cherry picked from commit 27c660c0)