Ray-caster: check layer count

In debug mode, show the layer count instead of the shaded image. This
reveals a bug: testing whether the hit depth is NaN doesn't actually
detect sphere misses, because `sphere_cast` returns -1 rather than NaN
to indicate a miss.

app-proto/inversive-display/src/inversive.frag

@@ -40,7 +40,7 @@ uniform vec2 radius;
 uniform float opacity;
 uniform float highlight;
 uniform int layer_threshold;
-uniform bool test_mode;
+uniform bool debug_mode;
 
 // light and camera
 const float focal_slope = 0.3;
@@ -151,6 +151,19 @@ void main() {
         }
     }
     
+    /* DEBUG */
+    // in debug mode, show the layer count instead of the shaded image
+    if (debug_mode) {
+        // at the bottom of the screen, show the color scale instead of the
+        // layer count
+        if (gl_FragCoord.y < 10.) frag_cnt = int(8. * gl_FragCoord.x / resolution.x);
+        
+        // convert number to color
+        ivec3 bits = frag_cnt / ivec3(1, 2, 4);
+        outColor = vec4(mod(vec3(bits), 2.), 1.);
+        return;
+    }
+    
     // sort the fragments by depth, using an insertion sort
     for (int take = 1; take < frag_cnt; ++take) {
         taggedFrag pulled = frags[take];