More Sprite stuff

[jalovatt]

Ideas that aren't feasible for 3.0.

• [ ] Rotation origin
  • Doable with the built-in params, but it might be more workable to rotate from 0,0 and then translate internally?
• [ ] Scale origin?
• [ ] Blending
  • [ ] Opacity
  • [ ] Modes
• Sprite-based element classes
  • Is there a reasonable way to make these a drop-in replacement for an element's graphics code? (Might require splitting elements into logic + theme first)
• [ ] loadFolder should optionally include subtrees
  • Probably just load them all "flat" with their relative paths rather than nesting imageFolder objects