Various tree fixes

[Csaboka]

Hi!

Today, I've been working on reproducing my "more consistent trees" patch for OpenTTD. The attached patch does the following:
- Trees can now be planted to bare land without making it grassy. The grass will grow below the tree at the usual pace.
- When you plant a tree in the desert, the tile doesn't become grassy just to revert to desert a second later.
- When a tree died in the desert, it left a snowy tile behind (until the periodic processing replaced it with a desert). The bug could be noticed in the vegetation map only; the graphics were the same.

I've also tried allowing cacti to spread in the desert, but it came out rather weird - some very big cactus patches developed in the desert, and that isn't too realistic. I've commented out that change, but it would be nice to allow cacti to spread, with different rules than ordinary trees.

The code changes are mainly straightforward: I'm copying the clear tile density for grass and rough land instead of having it constant zero, and adding the desert option to switch statements that forgot about it. The only gotcha is that tiles on rough land now have a constant density of 3 instead of a constant 0.

[athanasios]

Thanks for your "environmental concerns"!

[TrueBrain]

Added as r11244. Thanks a lot Finally someone fixed this annoying thingy