`add()`ing an element twice can result in bugs with element sorting

[anderoonies]

Describe the bug
Consider the following elementPool, where elements marked with o are .alive = true, x are .alive = false, and the vertical bar | represents elementPoolSize:

> add(a)
> add(b)
> add(c)
> add(a)

  o  o  o  o
[ a, b, c, a | ]

> remove(a)

  x  o  o  x
[ a, b, c, a | ]

// internal resort during redraw
// counts 2 deleted elements

  o  o   x  x
[ b, c | a, a ]

> add(a)

// increments elementPoolSize by 1

  o  o  o   o
[ b, c, a | a ]

> a.layer = 0

// internal resort during redraw

  o  o  o   o
[ a, a, b | c ]

// poor c :(