feat(examples): add the scene representation, camera and intersection

Sphere and plane records, camera ray generation from field of view and aspect
ratio, and the nearest-hit search across the scene.

examples/raytracer/include/rt_scene.h

@@ -0,0 +1,67 @@
+#ifndef RT_SCENE_H
+#define RT_SCENE_H
+
+#include "rt.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+    vec3 origin, dir;
+} ray;
+
+typedef enum { MAT_LAMBERT = 0, MAT_METAL = 1, MAT_LIGHT = 2 } mat_kind;
+
+typedef struct {
+    mat_kind kind;
+    vec3 albedo;
+    double fuzz;
+} material;
+
+typedef struct {
+    vec3 center;
+    double radius;
+    material mat;
+} sphere;
+
+typedef struct {
+    const sphere *spheres;
+    int count;
+    vec3 background;
+} scene;
+
+typedef struct {
+    vec3 origin, lower_left, horizontal, vertical;
+} camera;
+
+typedef struct {
+    double t;
+    vec3 point, normal;
+    material mat;
+    int front;
+} hit;
+
+camera camera_make(vec3 look_from, vec3 look_at, vec3 up, double fov_deg, double aspect);
+ray camera_ray(const camera *c, double u, double v);
+
+int scene_hit(const scene *s, ray r, double tmin, double tmax, hit *out);
+vec3 ray_colour(const scene *s, ray r, int depth, rng *rng_state);
+
+typedef struct {
+    const scene *sc;
+    const camera *cam;
+    unsigned char *rgb;
+    int width, height, samples, max_depth;
+    int row_next;
+    int threads;
+} render_job;
+
+/* Returns the number of worker threads actually used. */
+int render_run(render_job *job);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif