Remove focal_len parameter from camera (hard-code as 1.0)

include/camera.h

@@ -15,8 +15,8 @@ typedef struct {
 } camera_t;
 
 camera_t camera_init(
-    vec3_t pos, double focal_len, double fov, uint32_t img_width,
-    uint32_t img_height, unsigned samples_per_pixel);
+    vec3_t pos, double fov, uint32_t img_width, uint32_t img_height,
+    unsigned samples_per_pixel);
 
 void camera_render(
     const camera_t *camera, const obj_t *scene, unsigned scene_count,