[Impeller] SuboptimalKHR is not an exit condition. (flutter#45157)

Sometimes we can get SuboptimalKHR on swapchain aquisition, and if we dont treat that as success the app will exit.

Author: Jonah Williams

impeller/renderer/backend/vulkan/swapchain_impl_vk.cc

@@ -379,7 +379,8 @@ SwapchainImplVK::AcquireResult SwapchainImplVK::AcquireNextDrawable() {
     return AcquireResult{true /* out of date */};
   }
 
-  if (acq_result != vk::Result::eSuccess) {
+  if (acq_result != vk::Result::eSuccess &&
+      acq_result != vk::Result::eSuboptimalKHR) {
     VALIDATION_LOG << "Could not acquire next swapchain image: "
                    << vk::to_string(acq_result);
     return {};