Add Auto-Scroll variables for Scene backdrops, and animated backdrops #66

Is your feature request related to a problem? Please describe.
Currently, the only method to create animated or auto-scrolling backgrounds for scenes is the messy and ugly method of using an overwhelming amount of static AEmitters using your background sprite.

Describe the solution you'd like
Add variables under AddBackgroundLayer = SceneLayer that allows the player to set the auto-scroll speed of that background layer. something like AutoScrollX = 0, AutoScrollY = 0.

I'm not sure whether or not animated backgrounds are already actually supported, but giving scene backdrops the same animation system nearly everything else uses would greatly expand background variety.