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.

leaves.gdshader

@@ -14,9 +14,9 @@ void fragment() {
     // Use world Y position instead of UV
     float gradient_factor = (world_position.y + gradient_offset) / gradient_height;
     gradient_factor = clamp(gradient_factor, 0.0, 1.0);
-    
+
     vec3 final_color = mix(base_color.rgb, variation_color.rgb, gradient_factor);
-    
+
     ALBEDO = final_color;
     ROUGHNESS = 0.8;
     METALLIC = 0.0;