[AGG] New version of AGG coming to Debian/Ubuntu

[John H. <jo...@gl...>]

I am updating the libagg-dev Debian package to fix a packaging issue w.r.t
cross compilation. While I am at it I want to pick up some other changes:

1) Deliver a shared library version. There will also be a package libagg
that is just the runtime shared library support.
2) Pick up the patch request from matplotlib so that they can just depend
on libagg instead of embedding a version of agg with their additions and
bug fixes. I'm being selective about applying the matplotlib patch because
most of it seems to just be reverting our bug fixes and additons that
happened after their fork.
3) Bump the version to 2.6.0 because of the several compatible API
additions since AGG 2.4 started, skipping version 2.5. AGG 2.4 should
really have been AGG 3.0 because of the incompatibility with AGG 2.3.
4) Go all-in on the C++ standard library because it's 2018. All instances
of <math.h>, <stdlib.h>, <string.h>, etc are replaced with <cmath>,
<cstdlib>, <cstring>. All standard library functions are prefixed with
std:: Using the old C headers pollutes the global namespace with functions
and macros. With the C++ headers the functions typically are still at
global level, but the macros should not be.

-- john