The issue was caused by render result identifier only consist of scene name,
which could indeed cause conflicts.
On the one hand, there are quite some areas in Blender where we need identifier
to be unique to properly address things. Usually this is required for sub-data
of IDs, like bones. On another hand, it's not that hard to support this
particular case and avoid possible frustration.
The idea is, we add library name to render identifier for linked scenes. We use
library name and not pointer so we preserve render results through undo stack.