colormap.render is broken for some tile types

[moradology]

Tile requires a foreach method. For most tiles - ArrayTiles to be sure - a line scan from top left to bottom right is carried out. This is usually fine. ColorMap.render, however, assumes this behavior and can cause extremely hard to diagnose bugs as a result. For instance, the current PaddedTile implementation will cause tile data to be shifted up and to the left (as far as the tile boundary), pushing out any NoData values if ColorMap.render is called.

1. Maybe there's no good argument for colormaps having a render method and we should remove it
2. Perhaps the implementation of foreach violates expected semantics and should be corrected
3. Both 1 and 2?