draw: draw_distance for 3d spaces, proper positioning of skybox according to it, scenery demo on circle rasters

src/twn_camera_c.h

@@ -12,8 +12,9 @@ typedef struct Camera {
     Vec3 pos;        /* eye position */
     Vec3 target;     /* normalized target vector */
     Vec3 up;         /* normalized up vector */
-    float fov;       /* field of view, in radians */
     Vec2 viewbox[2]; /* othrographic aabb, ((left, right), (top, bottom)) */
+    float fov;       /* field of view, in radians */
+    float far_z;
 } Camera;
 
 Matrix4 camera_look_at(const Camera *camera);