[sldev] Webmap API change

[Ryan Williams (Which)]

Philip Rosedale wrote:
> I think that the problem is that you can only actually overlay
> transparent images in a browser using .PNG, right?  So that means that
> you can't really do it, because the only way to get anywhere near
> reasonable performance is the compression you get from JPEG.

The base images compress well with JPEG, but any overlay images you'd
care to generate will likely compress well with PNG.  E.g. Google's map
overlays are mostly transparent with solid blocks of color.  Also, while
it is unacceptable to have slowly-loading base tiles, it's not as bad to
have overlay tiles that take longer to download and pop in later, on
top.  Food for thought.  :-)  Glad you made provision for overlays, though.