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.

see_through.gdshader

@@ -0,0 +1,41 @@
+shader_type spatial;
+
+// Uniforms that can be controlled from GDScript
+uniform float model_height : hint_range(0.1, 100.0) = 10.0;
+uniform float transition_width : hint_range(0.01, 0.5) = 0.1;
+uniform vec4 base_color : source_color = vec4(1.0, 1.0, 1.0, 1.0);
+uniform float effect_strength : hint_range(0.0, 1.0) = 1.0;
+uniform sampler2D main_texture : source_color;
+uniform float metallic : hint_range(0.0, 1.0) = 0.0;
+uniform float roughness : hint_range(0.0, 1.0) = 0.5;
+
+varying vec3 world_position;
+
+void vertex() {
+    world_position = VERTEX;
+}
+
+void fragment() {
+    // Get the base texture color
+    vec4 tex_color = texture(main_texture, UV);
+    
+    // Calculate normalized Y position (0.0 at bottom, 1.0 at top)
+    // This uses the local vertex position - adjust based on your model's bounds
+    float normalized_y = (world_position.y + model_height * 0.5) / model_height;
+    
+    // Define fade boundaries - top 80% invisible, bottom 20% visible
+    float fade_start = 0.2; // Start fading at 20% from bottom
+    float fade_end = fade_start - transition_width;
+    
+    // Calculate position-based alpha
+    float position_alpha = smoothstep(fade_end, fade_start, normalized_y);
+    
+    // Blend between full visibility and position-based alpha based on effect strength
+    float final_alpha = mix(1.0, position_alpha, effect_strength);
+    
+    // Set material properties
+    ALBEDO = base_color.rgb * tex_color.rgb;
+    ALPHA = base_color.a * tex_color.a * final_alpha;
+    METALLIC = metallic;
+    ROUGHNESS = roughness;
+}