Если посмотреть в исходники GTK, то ругань эта идет вот из-за чего:
gdk_drawable_get_size
/**
* gdk_drawable_get_size:
* @drawable: a #GdkDrawable
* @width: location to store drawable's width, or %NULL
* @height: location to store drawable's height, or %NULL
*
* Fills *@width and *@height with the size of @drawable.
* @width or @height can be %NULL if you only want the other one.
*
* On the X11 platform, if @drawable is a #GdkWindow, the returned
* size is the size reported in the most-recently-processed configure
* event, rather than the current size on the X server.
*
**/
void
gdk_drawable_get_size (GdkDrawable *drawable,
gint *width,
gint *height)
{
g_return_if_fail (GDK_IS_DRAWABLE (drawable));
GDK_DRAWABLE_GET_CLASS (drawable)->get_size (drawable, width, height);
}
и gdk_drawable_get_depth:
/**
* gdk_drawable_get_depth:
* @drawable: a #GdkDrawable
*
* Obtains the bit depth of the drawable, that is, the number of bits
* that make up a pixel in the drawable's visual. Examples are 8 bits
* per pixel, 24 bits per pixel, etc.
*
* Return value: number of bits per pixel
**/
gint
gdk_drawable_get_depth (GdkDrawable *drawable)
{
g_return_val_if_fail (GDK_IS_DRAWABLE (drawable), 0);
return GDK_DRAWABLE_GET_CLASS (drawable)->get_depth (drawable);
}
Я с gtk не работал, но нутройм чую, что с drawable что-то не то, или нулевой или не так заполнен или еще чего.