We can vary the landscape a bit more by placing "fallen logs"
around the various forests. These decorations are quite fast
and will provide some gameplay value but are still more rare
than the corresponding trees, so they don't provide free
materials.

I've manually put the schematic as lua tables since these log
schematics are only 8 blocks. We vary the log lengths between
1 and 3 blocks by making the end blocks have a lower chance
of appearing.

Amount is varied by fill_ratio, except for acacia trees where
we reduce the scale, so that acacia logs show up near places with
acacia trees consistently.

Mushrooms are placed optionally on each log. We can't place
two different mushrooms on a log, so instead we opt to place
brown mushrooms on oak/appletree logs, brown mushrooms on
jungletree logs, and red mushrooms on pine logs. No mushrooms
are placed on acacia logs, as they occur in a dry biome,
savannah, and this adds a bit of biome diversity.