twn_camera: fix aspect of ortho projection

src/twn_camera_c.h

@@ -12,7 +12,7 @@ typedef struct Camera {
     Vec3 pos;        /* eye position */
     Vec3 target;     /* normalized target vector */
     Vec3 up;         /* normalized up vector */
-    Vec2 viewbox[2]; /* othrographic aabb, ((left, right), (top, bottom)) */
+    Vec4 viewbox;    /* othrographic aabb, ((left, right), (top, bottom)) */
     float fov;       /* field of view, in radians */
     float far_z;
 } Camera;