GtkIconHelper: Short circuit _gtk_icon_helper_get_size in some cases

GtkIconHelper: Short circuit _gtk_icon_helper_get_size in some cases

_gtk_icon_helper_get_size() is often used during size request and may
not necessary mean that the icon will be displayed immediately. In
many common cases we know the size without having to ensure a surface.
In many cases this means we can avoid loading an icon until needed, and
in the case of stateless IconHelpers such as GtkCellRendererPixbuf this
is very important as otherwise it will constantly be reloading icons
if the displayed set is larger than the in-memory icon cache.

https://bugzilla.gnome.org/show_bug.cgi?id=734651