/apps/demos/scenery: lock movement direction to a plane

apps/demos/scenery/scenes/ingame.c

@@ -83,6 +83,9 @@ static void process_ground_mode(State *state) {
     DrawCameraFromPrincipalAxesResult dir_and_up =
         draw_camera_from_principal_axes(scn->pos, (float)M_PI_2 * 0.8f, scn->roll, scn->pitch, scn->yaw);
 
+    dir_and_up.direction.y = 0;
+    dir_and_up.direction = vec3_norm(dir_and_up.direction);
+
     const Vec3 right = m_vec_norm(m_vec_cross(dir_and_up.direction, dir_and_up.up));
     const float speed = 0.18f; /* TODO: put this in a better place */