Reimplement AsPolygonSet without reference to another operation.

We're hitting a non-deterministic scala reflection bug which is causing problems with
this operation.  Soon, we need to remove the use of TypeTag altogether, but for now
this resolves the immediate issue.
Clean up rasterizer.

If polygon does not intersect raster extent, skip.  Additionally, this commit removes
an extraneous debugging println.
Resolve bugs in polygon rasterization.

This commit resolves a number of bugs in polygon rasterization,
primarily based on unsafe assumptions about the inputs of the
rasterizer.

Fixes #412, #407.
Merge pull request #420 from joshmarcus/topic/rasterization_fixes

Resolve bugs in polygon rasterization.