[Tilesathome] Best fix for missing captionless tiles?

[Alan Millar]

I'd like to fill in many of the missing captionless layer tiles.  I'm
mainly thinking of the ocean and coastal areas at the moment.

It seems to me more efficient to have a single blank area at the widest
scope/lowest zoom possible.  So the middle of the ocean could have a
single z4 blank tile instead of 65536 z12 tiles.  But that means a little
recursion when reading the tiles.  Which is costlier: more blank tile
records or more recursion?

Also, what happens when a blank tile has something placed over it?  For
example, if a z8 blank sea tile has an island placed in one corner, will
the blank tile stay in place "behind" the filled tiles, or will it be
deleted and the rest of the area filled with a bunch of "unknown type"
tiles?  I'm guessing the latter, but I don't want to assume.

I'm guessing that larger blank tiles are better, and partially blank areas
need to be filled with higher-zoom blank tiles.  Based on that I'm
considering writing a script to walk through the oceantiles.dat file
recursively and generate captionless-layer blank sea tiles to upload.

Any thoughts, comments, or objections?

- Alan