feat(examples): add the raytracer build file and core header

Nine targets: two translation units generated from scene files, a C++
consumer, a shared object built from the same sources as the archives, and an
optional zlib dependency.

examples/raytracer/include/rt.h

@@ -0,0 +1,43 @@
+#ifndef RT_H
+#define RT_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+    double x, y, z;
+} vec3;
+
+vec3 v3(double x, double y, double z);
+vec3 v3add(vec3 a, vec3 b);
+vec3 v3sub(vec3 a, vec3 b);
+vec3 v3mul(vec3 a, vec3 b);
+vec3 v3scale(vec3 a, double t);
+vec3 v3neg(vec3 a);
+double v3dot(vec3 a, vec3 b);
+vec3 v3cross(vec3 a, vec3 b);
+double v3len2(vec3 a);
+double v3len(vec3 a);
+vec3 v3norm(vec3 a);
+vec3 v3reflect(vec3 v, vec3 n);
+vec3 v3lerp(vec3 a, vec3 b, double t);
+
+/* Deterministic xorshift128+, so a render is reproducible. */
+typedef struct {
+    unsigned long long s0, s1;
+} rng;
+
+void rng_seed(rng *r, unsigned long long seed);
+double rng_unit(rng *r);
+double rng_range(rng *r, double lo, double hi);
+vec3 rng_in_sphere(rng *r);
+vec3 rng_unit_vector(rng *r);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif