As clip items aren't processed, they have no associated layer entries. This is
a problem when a clip item's underlying frame is the same as one that gets
merged into a container layer, as display-item data will be created, but no
layer entry will be added. This will cause it to be removed on the next
layer-build, and cause a full invalidation.
Fix this by adding an 'mIsMergedFrame' entry to DisplayItemDataEntry and setting
it on all merged frames in BuildContainerLayerFor. This property stops the
entry from being removed when it gets updated.
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