feat(rendering): distance fog and config-derived far plane

Assets/shaders/chunk.frag

@@ -2,6 +2,7 @@
 in vec2 vUv;
 in float vShade;
 in float vWorldY;
+in vec3 vWorldPos;
 
 out vec4 FragColor;
 
@@ -10,11 +11,25 @@ uniform float u_caveDarkStartY;
 uniform float u_caveDarkEndY;
 uniform float u_caveMinBrightness;
 
+uniform vec3 u_cameraPos;
+uniform float u_fogStart;
+uniform float u_fogEnd;
+uniform vec3 u_skyColor;
+
 void main()
 {
     vec4 texel = texture(atlas, vUv);
+
+    // Y-based cave darkness.
     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);
+    vec3 color = texel.rgb * vShade * brightness;
+
+    // Distance fog to the sky color, applied after cave darkness.
+    float dist = length(vWorldPos - u_cameraPos);
+    float fog = clamp((dist - u_fogStart) / (u_fogEnd - u_fogStart), 0.0, 1.0);
+    color = mix(color, u_skyColor, fog);
+
+    FragColor = vec4(color, 1.0);
 }