revisit solutions to coordinate overflow at high zooms

[springmeyer]

Clipping solves potential coordinate overflow and the resulting rendering oddities when drawing large polygons at high zooms (see #1075 and #1076).

But clipping is not always viable. We should revisit this problem to figure out if other solutions might exist.

Clipping may be off in this case: http://support.mapbox.com/discussions/tilemill/6834-land-and-water-inverting-at-zoom-level-17

[springmeyer]

looks like this is the blessed way to avoid overflows in agg, which I think we should use once I confirm no noticeable overhead

diff --git a/include/mapnik/agg_rasterizer.hpp b/include/mapnik/agg_rasterizer.hpp
index 1f93c55..5e82f03 100644
--- a/include/mapnik/agg_rasterizer.hpp
+++ b/include/mapnik/agg_rasterizer.hpp
@@ -31,7 +31,7 @@

 namespace mapnik {

-struct rasterizer :  agg::rasterizer_scanline_aa<>, mapnik::noncopyable {};
+struct rasterizer :  agg::rasterizer_scanline_aa<agg::rasterizer_sl_clip_int_sat>, mapnik::noncopyable {};

 }