Allows PickAny templates to be used in MultiBrushes, which can be
painted with random selection. This avoids the need to define various
separate MultiBrushes for each tile combination.
Note that all combinations count as part of a single MultiBrush, which
may influence selection weighting.
This MultiBrush feature is not presently used in OpenRA's official
mods, but is useful for unofficial mods using more general PickAny
templates.
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