Allow rewinding to a previous snapshot

[denisdefreyne]

Example:

compile '/assets/images/*.png' do
  filter :resize
  snapsnot :uncompressed
  write item.identifier.without_ext + '.png'

  filter :webp
  write item.identifier.without_ext + '.webp'
  rewind_to :uncompressed

  filter :avif
  write item.identifier.without_ext + '.avif'
end

[gpakosz]

As long as the compile block execution is serial, I believe this could go a long way.

This has the benefit of not messing with the concept of a :last snapshot.