Switch scene_f over to returning a SceneResult{dist, mat_idx}

examples/test/shaders/march_prolog.glsl

@@ -0,0 +1,8 @@
+struct SceneResult {
+	float d;
+	float mat_idx;
+};
+
+SceneResult min_sr(SceneResult a, SceneResult b) {
+	return (a.d < b.d) ? a : b;
+}

examples/test/shaders/test.frag

@@ -18,6 +18,7 @@ uniform float u_Time;
 out vec4 color;
 
 #include "utils.glsl"
+#include "march_prolog.glsl"
 #include "hg_sdf.glsl"
 #include "colormap_cool.glsl"
 
@@ -26,11 +27,11 @@ vec3 ray_dir(float fov, vec2 uv) {
 	return normalize(vec3(uv, z));
 }
 
-float scene_f(vec3 p) {
-	float b1 = fBox(p, vec3(1));
-	float s2 = fSphere(p + vec3(1), 1.0);
+SceneResult scene_f(vec3 p) {
+	SceneResult box = SceneResult(fBox(p, vec3(1)), 1.);
+	SceneResult sphere = SceneResult(fSphere(p + vec3(1), 1.0), 2.);
 
-	return fOpUnionRound(b1, s2, 0.5);
+	return min_sr(box, sphere);
 }
 
 vec3 estimate_scene_normal(vec3 p) {