Investigate functionalization of Spine 2D usage.

[bryanedds]

For the functional undo / redo support, perhaps the proper solution is to rewrite the C# Spine runtime in F# using immutable data structures. Alternatively, perhaps we can use the Prime primitives MutantCache or something similar but more custom-fit, presuming that the existing C# Spine runtime can be coerced to function as if time is a properly independent variable (which it currently looks like it isn't, but not totally sure).