[Xastir] Announcing tile support for OSM

[Tom Hayward]

> One item that will be useful at some point is the ability to grab to latest
> tile if there has been a change to it.

This is a good point. I haven't worked much with libcurl, but
command-line curl has the ability to look at a local file and generate
an HTTP If-Modified-Since header, so if the tile has not changed, the
server should respond only with 304 Not Modified. This is the curl
command:

curl -z local.html http://remote.server.com/remote.html

The download only gets performed if the remote file is newer than the
local copy.

If libcurl supports this, this feature could be augmented to the
current tiling functions pretty easily.

Tom KD7LXL