[Tilesathome] Lowzoom empty land/sea?

[Alan Millar]

I'm running lowzoom_composite on levels 8 to 11, and I've found something
strange.  I'm running it with the layers per the wiki, "tile captionless
caption", so it should produce the "tile" layer from the other two.

When the tile is completely blank land or sea, it does not produce "tile"
layer output.  It will create empty tiles for the captionless layer, but
not the "tile" layer.

You can see this by running

  lowzoom_composite.pl 222 157 8 12 tile captionless caption

and what I get is a lot of "captionless_" files created, and no "tile_"
files.  It seems to me that it should output both "captionless" and "tile"
layers as blank, which is what it does with non-blank tiles.

This must be a bug, right?  Is there some reason for this behavior?  I
can't figure out any, but I thought I better check first if someone knew
of a reason for it to be like this.

Assuming it is a bug, I'll patch it.

I also noticed that it does not composite captions onto blank tiles.  I
guess the assumption is that there must be an object for there to be a
caption.

Any feedback or advice is appreciated.  Thanks.

- Alan