Return a transparent tile if coord is out of bounds.

A layer can be configured with bounds, an optional dictionary of six tile boundaries to limit the rendered area. At the moment, however, a gray tile is returned if "coord" is out of bounds. Gray tiles, however, are not very useful if they are used as overlays or are part of a pixel sandwich: they will completely cover their background. For this reason, transparent tiles are returned if "coord" is out of bounds.