feat(rendering): add fog-distance darkness to chunk shaders

Assets/shaders/chunk.frag

@@ -1,13 +1,20 @@
 #version 330 core
 in vec2 vUv;
 in float vShade;
+in float vWorldY;
 
 out vec4 FragColor;
 
 uniform sampler2D atlas;
+uniform float u_caveDarkStartY;
+uniform float u_caveDarkEndY;
+uniform float u_caveMinBrightness;
 
 void main()
 {
     vec4 texel = texture(atlas, vUv);
-    FragColor = vec4(texel.rgb * vShade, 1.0);
+    float t = clamp((u_caveDarkStartY - vWorldY) /
+                    (u_caveDarkStartY - u_caveDarkEndY), 0.0, 1.0);
+    float brightness = mix(1.0, u_caveMinBrightness, t);
+    FragColor = vec4(texel.rgb * vShade * brightness, 1.0);
 }