gtkiconhelper: also apply icon effects when the source is a cairo surface. Fixes #1542

gtk_css_icon_effect_apply() was only called on all icon types that were converted to cairo surfaces but not on cairo surfaces themselves.

This was for example visible when moving a GtkImage from GdkPixbuf to cairo_surface_t and gtk_widget_set_sensitive() on the image no longer having any effect.

To fix this copy the surface and apply the effect in case there is an effect active. Otherwise just use the surface as is.