This function is called once for each block of bytes that is "written"
by [gdkpixbuf.pixbuf.Pixbuf.saveToCallback].
If successful it should return TRUE; if an error occurs it should set
error and return FALSE, in which case [gdkpixbuf.pixbuf.Pixbuf.saveToCallback]
will fail with the same error.
Save functions used by gdkpixbuf.pixbuf.Pixbuf.saveToCallback.
This function is called once for each block of bytes that is "written" by [gdkpixbuf.pixbuf.Pixbuf.saveToCallback].
If successful it should return TRUE; if an error occurs it should set error and return FALSE, in which case [gdkpixbuf.pixbuf.Pixbuf.saveToCallback] will fail with the same error.