Fix dereferencing a dangling pointer in WorldNode

[jslee02]

In clearUnusedNodes(), if an unused node doesn't have any holders that have the ownership, it becomes dangling pointers since WorldNode holds nodes as raw pointers.

In clearUnusedNodes(), an unused node is possible to hold a dangling pointer to ShapeFrame. This can happen because ShapeFrameNode doesn't have the ownership of ShapeFrame. This leads to dereferencing a dangling pointer in the logic of checking whether the node is shadowed or not.

This PR changes the behavior of the node deletion logic to remove the access to the internal ShapeFrame pointer. Instead, it now unconditionally removes the node from both of shadowed group and unshadowed group. I believe this is safe because OSG will ignore the attempt of removing a non-existing node in the group.

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[jslee02]

It seems the actual dangling pointer is the ShapeFrame in ShapeFrameNode (not the ShapeFrameNode itself since the group has the ownership). Will update this PR if needed.

[codecov]

Codecov Report

Merging #1311 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   57.18%   57.18%           
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  Files         366      366           
  Lines       27023    27023           
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  Hits        15454    15454           
  Misses      11569    11569