[png-mng-misc] Masks AGF concept - need or not?

[Alexey S. <ac...@gm...>]

I think add alpha masks support for AGF. But does not know, need or not?
I have some suggestions about masks. By one version, add to draw operation
and add property
"use_mask = 0 / 1" and
"channel_for_alpha = 0 / 1 / 2 / 3" and
"mask_operation = mult / replace".
I know that masks applying for on boundary of frame.
What you think about masks for AGF format?