Implements tree visibility occlusion system.

Adds tree fading based on camera line of sight.
Trees now fade out when they obstruct the player's view,
improving visibility.

Also includes several fixes:
- Fixes tree distribution.
- Adds a see-through shader.
- Adds camera and occlusion scripts.

project.godot

@@ -64,6 +64,10 @@ jump={
 ]
 }
 
+[layer_names]
+
+3d_physics/layer_4="TreeVisibility"
+
 [network]
 
 limits/debugger/max_chars_per_second=1000000