message: Fix frame counter in async mode
diff: @@ -295,6 +295,7 @@ struct renderer : viewer_type
  mouse::pos mouse_pos;
  visionaray::frame_counter counter;
  + double last_frame_time = 0.0;
  gl::bvh_outline_renderer outlines;
  gl::debug_callback gl_debug_callback;
  @@ -743,7 +744,7 @@ void renderer::render_hud()
  ImGui::SameLine();
  ImGui::Text("B: %5.2f", rgba.z);
  - ImGui::Text("FPS: %6.2f", counter.register_frame());
  + ImGui::Text("FPS: %6.2f", last_frame_time);
  ImGui::SameLine();
  ImGui::Spacing();
  ImGui::SameLine();
  @@ -1012,6 +1013,8 @@ void renderer::render_impl()
  }
  #endif
  + last_frame_time = counter.register_frame();
  +
  #if VSNRAY_COMMON_HAVE_PTEX
  // if (mod.ptex_textures.size() > 0)
  // {
  