Add minimal dungeon drawing code

frontend/src/shaders/rainbow.frag

@@ -0,0 +1,18 @@
+vec3 rainbow(float level) {
+	float r = float(level <= 2.0) + float(level > 4.0) * 0.5;
+	float g = max(1.0 - abs(level - 2.0) * 0.5, 0.0);
+	float b = (1.0 - (level - 4.0) * 0.5) * float(level >= 4.0);
+	return vec3(r, g, b);
+}
+
+vec4 smoothRainbow(float x) {
+    float level1 = floor(x*6.0);
+    float level2 = min(6.0, floor(x*6.0) + 1.0);
+    
+    vec3 a = rainbow(level1);
+    vec3 b = rainbow(level2);
+    
+    return vec4(mix(a, b, fract(x*6.0)), 1.0);
+}
+
+fragmentColor = smoothRainbow(getDistance(position) / 30.0 + 0.5);