Log Message:
Update to 2.40.15

Changelog:
Version 2.40.15
- Apologies for the lack of 2.40.14.  I mistakenly tagged the
  repository before updating the NEWS file.
- librsvg now uses the Contributor Covenant Code of Conduct,
  version 1.4, to which all contributors and maintainers are expected
  to abide. Please see the code_of_conduct.md file for details.
- Chun-wei Fan fixed builds on Visual Studio pre-2012.
- Fixed bgo#759084 - Don't crash when filters don't actually exist
  Fix by Benjamin Otte.
- Javier Jardón updated our autogen.sh to use modern autotools.
- Fixed bgo#761728 - Memory leak in the PrimitiveComponentTransfer
  filter.  Fix by Ron Hopper.