I like very much this contribution. Thanks Emre !
I just need more control on the way the vegetation and rocks are set, so I am going further on your script by adding a bitmap mask where

• red channel is for the trees,
• green is for the bushes 1,
• blue is for the rocks,
• alpha could be for the bushes 2

Channel value (0 -> 255) will be used for the density.
There is still random settings for natural effect.
In the black area of the bitmap, nothing is set.

I am getting first results with the red channel, as you can see here :

Here is the bitmap:

Another example