librsvg: update to 2.40.18.

Version 2.40.18
- bgo#783835 - CVE-2017-11464 - Fix division-by-zero in the Gaussian
  blur code.
- Fix other cases of division-by-zero on fuzzed SVG files.
- Don't crash on invalid transformation matrices.
- Support Visual Studio 2017; generate .pc files for Meson on Windows -
  Chun-wei Fan.

Version 2.40.17
- bgo#778666 - Use our own thumbnailer specification file, so
  gnome-desktop-thumbnailer can generate thumbnails outside of the
  calling process (Jeremy Bicha).
- Removed some redundant code.
- Windows build fixes - Chun-wei Fan.

Version 2.40.16
- Chun-wei Fan added support for building the introspection files under MSVC.
- Fixed bgo#760262 - Make the zooming options in rsvg-convert(1) work again
  for scaling the resulting image.  Fix by Menner <mix@gmx.org>.
- Fixed bgo#764808 - Wikipedia generates equations as SVGs and renders
  them, but uses fill="currentColor".  Since we don't let callers
  specify a starting state for CSS, we need to start with opaque black
  as the default current color.
- Added documentation for how to replace the deprecated
  rsvg_handle_set_size_callback().

(wiz)