Upgrade agg to SVN version

As a first step toward doing image interpolation in floating-point, and having (optional) floating-point rendering buffers for more accurate alpha blending, this upgrades our included copy of Agg to the current SVN version here: http://sourceforge.net/p/agg/svn/HEAD/tree/

This version of Agg is a fork of Agg 2.4, so it is still under a BSD license.

A few caveats: There is a regression in the alpha blending on RGBA 32-bit (8-bit-per-plane) buffers caused by doing the alpha blending calculation in 8-bits rather than 32-bits as it was before.  I've communicated on the Agg mailing list about a solution, but I can't find the "right way" to do it.  So, instead, I've included the one-off solution that uses the old calculation from Agg-2.4 verbatim.

The core Agg developers seem to see no need for releasing tagged versions of Agg.  (Again, discussed on the mailing list).  This may be a problem for some packagers, since matplotlib can no longer be built against the "system" Agg.  Currently, Fedora uses a system Agg for its matplotlib package, but Debian, Arch, MacPorts, and Homebrew do not.
It's left the same as Agg-2.4 (1024), but is no longer hard coded and may be changed from the outside.  So instead of patching the original code as we did prior to this PR, we pass our cell_block_limit in as a parameter in `_backend_agg.cpp` on the line:

```
theRasterizer(4096)
```