pHYs support

[clord]

It would be nice to have api support for generating the pHYs section, as it is part of the PNG Specification.

It's possible to do it manually if there is already a chunk:

stream.each_chunk do |c|
  if c.type == 'pHYs'
    c.content = [5906, 5906, 1] .pack('N2C')
  end
end

But it's not as easy to insert this node into a fresh PNG. I plan on inserting a ChunkyPNG::Chunk::Generic into #other_chunks, but this feels really a tad dirty. I propose that Datastream have ChunkyPNG::Chunk::Phys and a corresponding phys_chunk accessor.

[wvanbergen]

👍 this would be a nice addition.

It's unlikely I will have time to work on this anytime soon though, but feel free to ask me to help if you are willing to take a stab at it.