Set the priority of group-layer projections according to the group
layer's depth, so that top-level groups have a priority of 1
(compared to a priority of 0 for the image projection), and nested
groups have a priority one greater than their parent (in other
words, shallower groups have higher priority than deeper groups,
all of which have lower priority than the image.)
This makes pass-through groups much faster, in particular.
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