Command that retrieves raster data. The result contains a list of raster tiles. Basic assumption is that
raster data is tiled in a pyramid structure based on to the layer's maximum extent and tile level. The optimum tile
level for the given scale is determined first. Once the tile level is known, it should be a piece of cake to
determine the indices. The client can cache as many tiles as it likes.
Pieter De Graef, Jan De Moerloose, Joachim Van der Auwera