That's kinda weird but allows us to delete the texture case from
GtkIconHelper and GTK_IMAGE_TEXTURE from the GtkImageType enum.
And it doesn't cause any other problems because the cell renderer
can't deal with paintables - otherwise it would mirror GtkImage and have
a "paintable" property instead.
Instead of loading them into surfaces (which we want to get rid of), we
load into textures.
In fact, we introduce a new paintable subclass called a GtkScaler that
takes care of tracking scaling.
This also ideally gets rid of an extra conversion once renderers learn
to render textures directly.
These are no longer used, instead we always covert to surface as
early as possible and drop the pixbuf.
This means we never store both the pixbuf and the surface at
for any longer time, which is wasteful. Also, its one step further
to drop GdkPixbufs from generic use in our APIs.
The size of icons is a property that is relevant to who is rendering the
icon, not to the icon itself.
Example: Starting a DND operation from an entry icon should cause the
icon to resize (from the entr icon's size to the DND icon size).
