Raster Field in Features

I know that @pnorman @pramsey and @ShibaBandit all commented in #39 about the experimental state of rasters. Here is the best clarification I can offer:

A decoder using v2 can simply ignore the raster field, we realize that a more detail specification for rasters is needed. We currently aren't sure how we think rasters should best be put into this field and some research/experimenting is required. We did want to include the field however, as we will be working in the mapnik-vector-tile repository to expand the capabilities for rasters in vector tiles. Additionally, @springmeyer has told me that some groups have already started to experiment with this as a feature. The goal is that we will have this as an experimental field for version 2 and it will lay the groundwork for the community to work on how specifically how the raster data will be stored.

Start with the discussion in issue #39 and the comments referenced here, then review the raster experiments in the mapnik-vector-tile repository. Done means the community has agreed on a detailed specification for how raster data is stored in the experimental version 2 field.