Update `TileExploder` to handle "proj_raster"s

[metasim]

Should use the same trick other Expresssions do to handle both top-level Tile types and ones nested in a TileContext.

[vpipkt]

Its a problem for users: https://gitter.im/locationtech/rasterframes?at=5d68e00f5c4d6813a52d0921