Since setting a clip is mandatory for almost all widgets, we can as well
change the size-allocate signature to include a out_clip parameter, just
like GtkCssGadget did. And since we now always propagate baselines, we
might as well pass that one on to size-allocate.
This way we can also make sure to transform the clip returned from
size-allocate to parent-coordinates, i.e. the same coordinate space
priv->allocation is in.
GtkGizmo is the easiest possible widget to implement. It does nothing
except give its creator a way to control measure/size-allocate/snapshot,
so it can be used in a variety of use cases.
