pixops: Be more careful about integer overflow

Our loader code is supposed to handle out-of-memory and overflow
situations gracefully, reporting errors instead of aborting. But
if you load an image at a specific size, we also execute our
scaling code, which was not careful enough about overflow in some
places.

This commit makes the scaling code silently return if it fails to
allocate filter tables. This is the best we can do, since
gdk_pixbuf_scale() is not taking a GError.

https://bugzilla.gnome.org/show_bug.cgi?id=752297