Pre-compute per-observer values (root node, root paintable, root
bounds, is_implicit_root, root_is_element) once before the per-target
loop instead of recomputing them for each target.
Add intersection_root_node() which returns GC::Ref<DOM::Node> without
creating GC::Root wrappers. Previously, intersection_root() was called
multiple times per target (in the skip condition check, in
compute_intersection, and again in root_intersection_rectangle inside
compute_intersection), each call creating GC::Root objects that require
heap allocation via new RootImpl.
Pass the pre-computed root bounds and root paintable into
compute_intersection instead of having it call
root_intersection_rectangle() and intersection_root() internally.
For an observer watching N targets, this eliminates roughly 4N heap
allocations per frame.
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