[CREATE] [OpenRaster] improve interpretation of layer group offsets

[Andrew Chadwick]

There is some confusion in the GIMP bug tracker about how OpenRaster
layer stack offsets apply:

https://bugzilla.gnome.org/show_bug.cgi?id=760348

Do we need some clarifying language in
https://wiki.freedesktop.org/www/Specifications/OpenRaster/Draft/LayersStack/
?


I propose:

---------------8< -----------------

x and y attributes

These attributes define the offset of the layer or stack relative to
its parent. These are signed integers with a default value of 0. These
attributes must not be set on the root stack element: the root stack
has zero offset.

The offset of a layer or a stack must be calculated relative to the
calculated offset of its parent stack. For the example above, the
"OpenRaster Logo" layer has coordinates (x=15, y=5) in the image's
coordinate system.

The X axis is oriented from left to right, and the Y axis is oriented downwards.

--------------->8 -----------------

It's probably worth having a para about Y having the usual computer
graphic interpretation too, so I added that.


-- 
Andrew Chadwick