After this commit, spigot now creates a deep copy of the
itemmeta's persistent data container when the itemmeta
instance is cloned.
This change fixes the bug that, after cloning itemmeta, the container
instance the cloned meta would point to was equal to the original one.
This resulted in two itemmeta instances sharing a single persistent
container.
CraftBukkit (and Minecraft as well in some cases, such as when getting
an item from the creative inventory menu) will omit the Damage tag when
it is zero. However, minecraft will add the tag in some situations
nevertheless, such as when loading the ItemStack, or when explictly
setting the item undamaged.
These items (with and without the Damage tag for undamaged items) will
be considered as different by minecraft and CraftBukkit in various
situations, even though they should not. In CraftBukkit these items will
actually only be considered unsimilar if the items' metadata is not
'empty' (if it contains other additional metadata, such as enchantments,
etc.).
If the item's tag is empty after removing the Damage tag, it gets
completely removed. The setRepairCost function was adapted to behave in
the same way (removal of the tag if it becomes empty).
The Minecraft implementation of spawn eggs is able to construct an entity using all data that is present in the save format, however since the Bukkit API has no such way to construct an entity unattached to a world, and it appears creating such a way is a very challenging task, the decision was instead made to add this API now that 1.11 has entities which may not be represented by data values.
In the future it may be possible to implement a more expanded API cognate with this one.
