Move SnapToGrid to engine

[ElectroJr]

This adds content's SnapToGrid extension methods to SharedTransformSystem.

This PR also changes the static IoCManager.Resolve methods so that they don't throw an NRE in uninitialized threads even when you pass non-null values (fixes #6274)

[PJB3005]

What is the use case for this? This kind of feels like a high-level helper function and I'm not sure it's appropriate to have in engine.