Previously, papyrus was limited to the savanna biome.
Rainforest is a suitable habitat for papyrus.
Makes papyrus a little less difficult to find.
Move swamp jungletrees into a separate registration, to allow
applying distribution by perlin noise to keep these away from
papyrus areas.

Fix error: Remove non-functional 'rainforest swamp' from the
biome list for the jungle log decoration. It is not placed in
swamps due to the 'y max/min' and 'place on' parameters.

Implement drag force according to the equation:
drag_force = drag_coefficient * speed ^2
Also add a small constant force to implement friction force.

These are unrelated to the Player API and player object.
All other GUI/HUD code and textures are in default.
All other engine hardcoded sounds are in default.
The player_api mod is highly likely to be used unchanged in new games,
so logical grouping of content will help the creation of new games.

No longer use the unsuitable steel door/trapdoor sound.

Avoids player-placed leaves decaying when building while sneaking, which is common.

Register using the 'doors' mod API.

Increase noise resolution from 4 to 2 nodes for a higher quality
distribution.
Retune noise parameters to compensate for using eased noise.

Add an option, default disabled, to extend the collision boxes upwards.

broken by bb9279cc

default:river_water_source can also create sound if desired as
rivers are considered to be flowing water.
A simple mod for now, with the intention to later use new engine
environmental sound features if/when they appear.