message: Vulkan: Fix layout of destination image of T2T copies
  Bug=dawn:18
diff: @@ -348,8 +348,10 @@ namespace dawn_native { namespace vulkan {
  VkImageCopy region = ComputeImageCopyRegion(src, dst, copy->copySize);
  + // The dstImage is written to so the Dawn guarantees make sure it is in the
  + // TRANSFER_DST_OPTIMAL layout
  device->fn.CmdCopyImage(commands, srcImage, VK_IMAGE_LAYOUT_GENERAL, dstImage,
  - VK_IMAGE_LAYOUT_GENERAL, 1, &region);
  + VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL, 1, &region);
  } break;
  case Command::BeginRenderPass: {
  