Remove FIXME sections from code
Remove FIXME sections from code

One section can't be improved from what I can gather,
because we have to iterate the number of bits per pixel in advance.

The output section has been refactored to pull the block writing code
into a separate function.

Fixes #19

shssoichiro/oxipng,shssoichiro/oxipng,shssoichiro/oxipng