png for hmap replace

This is just an replacment format for hmap, it's not for unifying fractals to go with it.
Motivation was #1992, so we can easily mod heightmaps with current earth and moon fractals. (even tho they should be unified I agree)

Here is what earth and moon currently look like using th new png heightmaps.
Patches are welcome if it sucks. :)

![Alt Text](https://dl.dropbox.com/u/11319604/pnge.png)

![Alt Text](https://dl.dropbox.com/u/11319604/pngm.png)