It allows to specify the resize mode of the paintable inside the
GtkPicture allocation. This also deprecates the keep-aspect-ratio
property.
Fixes#5027.
Have square images in the following sizes:
* 20
* 100
* 150
* 200
* 300
and place them in a can-shrink Picture allocated at the sizes:
* 200x100
* 100x200
and set align to center/center.
That's 10 combinations and they should all do the right thing.
