add zoom option to camera to work around the orthographic camera

src/twn_camera_c.h

@@ -13,6 +13,7 @@ typedef struct Camera {
     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)) */
 } Camera;
 
 Matrix4 camera_look_at(const Camera *camera);