* Cactus detection code follows pattern set in #3996
* Pickups are now destroyed on cactus contact
* Add cactus detection and destruction to Exp Orbs
Remove checks for IsExpOrb() in cEntity::Tick()
Exp Orbs do not call super::Tick() and so this condition was
pointless.
* Check for intersection between placed blocks and entities.
+ Implemented GetPlacementCollisionBox, to permit custom placement collision boxes for blocks.
* Factored block-entity placement checking into another function in cPlayer.
- Removed vector min/max functions
* Use GetWorld to get the world in DoesPlacingBlocksIntersectEntity.
+ Added block height checks, allow different cEntity subclasses to decide whether they will prevent block placement.
This was mostly done automatically and then visually inspected for obvious errors.
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