Danger/skull animation

[jakubg1]

• When the spheres reach a danger point, skulls should appear behind the balls on the path and they should flash towards the end point.
• The skull should be, obviously, animated.
• Since the skull center is always its endpoint, it can be rendered along with the path itself.
• If the player dies, a completely separate skull effect with eating the frog would appear. The source seems to be always the first path specified; no detection which path has breached the endpoint seems to be made.

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• bf19d02 Implement base for skull rendering
• 05631b4 Refine animation (make it smoother)
• Smaller skulls on the path that start from the danger zone and glow towards the skull
• Implement fire additive effect around skull frame
• Add death animation

[ShamblesSM]

(Edited because tasks don't apply to comments)

I've pushed a commit that contributes to this issue (096d353). I forgot to mention it there though.

Edit 2: Force pushed