Compute a concrete size for the `GdkPaintable`.
Applies the sizing algorithm outlined in the [CSS Image spec](https://drafts.csswg.org/css-images-3/
default-sizing) to the given
this. See that link for more details.
It is not necessary to call this function when both
specified_height are known, but it is useful
to call this function in GtkWidget:measure implementations to compute the other dimension when only one dimension is given.
the width this could be drawn into or 0.0 if unknown
the height this could be drawn into or 0.0 if unknown
the width this would be drawn into if no other constraints were given
the height this would be drawn into if no other constraints were given
will be set to the concrete width computed.
will be set to the concrete height computed.