Raster support.

Addition of RasterDataset and supporting classes for reading raster data.
First cut at rendering raster data.

- Updated BaseRenderer to include common raster rendering routine.
- Add canRender* methods to BaseRenderer to define child rendering capabilities.
- Updated GTRenderer to draw rasters.
Raster implementation based on GDAL.

Includes a specific GeoTIFF driver class. Fixes #3.