Include latitude and longitude in tile

[gridcell]

In the past, I've wanted to know the center point (potentially bounding box) of the tile and would be interested in seeing that in the tile output.

[captaincoordinates]

@gridcell is it sufficient to include the centre point of a tile in its HTTP response headers, or do you also want to see this printed in the image? Bounding box is currently provided in a HTTP response header but not included in the image.