rmgen heightmap image import

[elexis]

While it is a rightfully demanded feature to open terrain maps created in atlas from a random map script, it would also be handy to open an image file of a heightmap directly from rmgen without creating a PMP file thereof.

The rmgen script can then be tought to interpolate, set the waterheight and so forth. This way one can keep the original data source and if needed replace it while the code and map generation still works. (It means if such a replacement is needed or benefitial, artistic value of the terrain is not lost).