Bug/Improvements: Background Group API

[pushfoo]

What should be added/changed?

• [ ] Fix the divide by zero error when a layer's depth is 0 in ParallaxGroup
• [ ] Add support for an initial offset to layers / groups
• [ ] An INFINITE_DEPTH constant to pin things in the distance
• [ ] A helper function to sync the offset & background position from a camera or coordinate

What would it help with?

1. Usability for beginners
2. General API consistency

[DragonMoffon]

Move to 3.1 with other background stuff