present_mode: set to FIFO is not available

When a present mode is not available for some reason, then vulkan fallsback
to FIFO. So we just say it's FIFO and call it day.

src/vulkan.cpp

@@ -1541,6 +1541,7 @@ static VkResult overlay_CreateSwapchainKHR(
             SPDLOG_DEBUG("Present mode: {}", HUDElements.presentModeMap[HUDElements.cur_present_mode]);
          else {
             SPDLOG_DEBUG("Present mode is not supported: {}", HUDElements.presentModeMap[HUDElements.cur_present_mode]);
+            HUDElements.cur_present_mode = VK_PRESENT_MODE_FIFO_KHR;
          }
       }
    }