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