icontheme: Don't assume a symbolic icon size of 16x16

icontheme: Don't assume a symbolic icon size of 16x16

If the symbolic icon has other size than 16x16, the embedder
SVG that overrides colors would still force that size, resulting
in clipping instead of resizing. So fetch the original pixbuf
size the first time a symbolic icon is requested for a GtkIconInfo,
and use that size for the embedder SVG so it can be scaled properly
afterwards.

https://bugzilla.gnome.org/show_bug.cgi?id=677567