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 
I think you can drop the while loop here which will end up being slow if the datasource is very large (you'll end up loading every single feature).

If the very first call to `featureset.next()` you can assume the datasource is empty -- otherwise you've grabbed the first feature successfully which is an indication that it's not empty.