Created CanBuildFrom from ValueSource[_] => RasterSource

If you map a ValueSource to a Raster, you should get a RasterSource back. For example, 

```scala
ValueSource[Polygon](somePolygon).map(Rasterizer.rasterizeWithValue(_, someRasterExtent) { x:Int => 1 })
```

should give a RasterSource back. This commit makes it so.

Also fixed the .info for RasterSource's that are in memory...if there's no valid LayerId, then we shouldn't be asking the Catalog for information about the layer.