create weather controller and daynight controller

core/daynight/snow.gdshader

@@ -0,0 +1,19 @@
+shader_type spatial;
+render_mode blend_mix, depth_draw_opaque;
+
+global uniform float global_snow_amount;
+
+uniform vec4 snow_color : source_color = vec4(0.95, 0.98, 1.0, 1.0); 
+
+void fragment() {
+    vec3 world_normal = (INV_VIEW_MATRIX * vec4(NORMAL, 0.0)).xyz;
+    float up_dot = dot(world_normal, vec3(0.0, 1.0, 0.0));
+    float snow_mask = smoothstep(1.0 - global_snow_amount, 1.1 - global_snow_amount, up_dot);
+    snow_mask *= step(0.01, global_snow_amount);
+
+    ALBEDO = snow_color.rgb;
+    ALPHA = snow_mask;
+    
+    ROUGHNESS = 0.3; 
+    SPECULAR = 0.5;
+}