Use `BoundingSphere` natively

[Sezzary]

• Fully replaced legacy SPHERE struct with DXTK's BoundingSphere.
• Removed CreatureSpheres and LaraSpheres globals.
• GetSpheres() returns a vector instead of filling an output argument.

To test:

• Ahmet flames.
• Shatters.
• Collision with creatures.
• Killing of creatures.

When reviewing code, look out for any changes that may not be the equivalent of what was there before. This is a simple replacement refactor, nothing fancy.

To do:

• Fix pushes.