Tree spawn chances fix

This fixes the tree spawn chances sliding error.

(Basically, with code like: 
if (one_in(200)): apples
else if (one_in(200)): oranges...etc
the later items will have lower chances than those specified.
discussion about it here:  #8896)

fixed by increasing the later items spawn chances (calculating the amount needed to keep their actual spawn chances correct)

The forest  code now takes the 'one_in chances' & 'terrain to spawn' from an array, so i'm sure this can be easily JSONised later on.