Mapnik

Handle Mapnik version update. The mapnik update [brought to light some possible issues with GPX sources](https://github.com/mapbox/mapnik-omnivore/issues/25). 

For example, the old mapnik-omnivore code wasn't throwing errors for GPX sources with layer extents of [0,0,0,0]. It also wasn't taking into account all possible gpx layers. I added `track_points` and `raster_points`

I also updated test fixtures to be more readable and to handle more floating point issues with raster band properties.

/cc @yhahn