GdkPixmap vadility check

[WDraxinger darkstargames de]

I need a function to check, if a pixmap is still valid,
which I was gdk_pixmap_unref()-ing.
Thats, because I'm writing an material editor for my
3D engine, and each render layer is shown through a
small thumbnail in a list. Since all values in the
list are dynamic, but may refer to the same texture
I invented a small helper class which manages the texture
ID, the Pixmap and so on. The pixmap itself is of course
ref-/unref-ed. But with the destruction of the last class
instance that refers to the corresponding pixmap
a bit more cleanup work must be done. So I search for
an easy way to obtain the validity of a pixmap.

Thanks.
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