Allow -1 to be passed for auto focal length

src/camera.c

@@ -12,6 +12,9 @@ camera_t camera_init(
     vec3_t pos, vec3_t target, double fov, uint32_t img_width,
     uint32_t img_height, double aperture, double focal_len)
 {
+	if (focal_len = -1)
+		focal_len = vec3_len(vec3_sub(target, pos));
+
 	const double fov_rad = M_PI * fov / 180.0;
 	const double aspect = (double)img_width / (double)img_height;
 	const double viewport_height = focal_len * tan(fov_rad / 2);