This function is only used by `GtkWidget` subclasses, to assign a size, position and (optionally) baseline to their child widgets.
In this function, the allocation and baseline may be adjusted. The given allocation will be forced to be bigger than the widget's minimum size, as well as at least 0×0 in size.
For a version that does not take a transform, see [method@Gtk.Widget.size_allocate].
New width of this
New height of this
New baseline of this, or -1
Transformation to be applied to this