ImageIO Format PNG: cleanup
ImageIO Format PNG: 8bit: avoid additional buffer copying.
Let libpng remove alpha.
ImageIO Format PNG: 16bit: avoid additional buffer copying.
Let libpng remove alpha and swap bytes.
ImageIO Format PNG: refactor common code
Merge pull request #693 from LebedevRI/imageio-format-png-update

Imageio format png update
Merge branch 'master' of https://github.com/darktable-org/darktable