[Bf-codereview] Alpha premul pipeline cleanup (issue 7018048)

[brechtvanlommel at gmail.com]

* Are we handling 16 bit PNG images correctly, are they convert from
straight to premultiplied on load and vice versa for loading?
* For TIFF we should also read/write the metadata about the alpha type?
* Is the image painting code working correct for premul float and
straight byte?

https://codereview.appspot.com/7018048/