Physics adjustments for z-levels

[metalgearsloth]

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Even if we do z-levels on content, thinking about it engine is gonna need some changes.

The biggest issue is we need to support cross-map joints to connect grids across z-levels via welds. This probably entails killing sharedphysicsmapcomponent and having awake physics bodies use a component instead of being a property. You then move contacts to its own component as they're still map based, but the physics island solver will be global now.