Set level count for wrapped backbuffer

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1965533002

Review-Url: https://codereview.chromium.org/1965533002
diff --git a/tools/viewer/sk_app/VulkanWindowContext.cpp b/tools/viewer/sk_app/VulkanWindowContext.cpp
index f21ec9c..d2ea2be 100644
--- a/tools/viewer/sk_app/VulkanWindowContext.cpp
+++ b/tools/viewer/sk_app/VulkanWindowContext.cpp
@@ -261,6 +261,7 @@
         info.fImageLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;
         info.fImageTiling = VK_IMAGE_TILING_OPTIMAL;
         info.fFormat = format;
+        info.fLevelCount = 1;
         desc.fWidth = fWidth;
         desc.fHeight = fHeight;
         desc.fConfig = fPixelConfig;