Avoid array insertion in iterative refinement

[schillic]

In the "iterative refinement" of polygons we use array insert!, which is not efficient (in practice it might be okay because we start at the front and walk to the end).
An efficient alternative is to have two arrays, one for finished refinements and one for unfinished ones:

initialize with box direction approximations
[] [a b c d]
turn around initial array
[] [d c b a]
# actually just initialize in opposite order, but for clarity this is described in this way
pick a
[] [d c b]
refine a to (d, e)
add them in the opposite order
[] [d c b e d]
pick d
assume it is not refinable, so add it to the left
[d] [d c b e]
pick e
[d] [d c b]
assume it is not refinable, so add it to the left
[d e] [d c b]
...