[hgiVulkan] Fix for HgiVulkanBlitCmds::CopyTextureGpuToCpu. Vulkan on…

…ly expects

one aspect when copying an image to buffer, so if the image is a depth-stencil
image, assume we wish to copy the depth aspect.

See #2852

(Internal change: 2316631)

pxr/imaging/hgiVulkan/blitCmds.cpp

@@ -97,12 +97,23 @@ HgiVulkanBlitCmds::CopyTextureGpuToCpu(
     size.depth = texDesc.dimensions[2] - depthOffset;
 
     VkImageSubresourceLayers imageSub;
-    imageSub.aspectMask =
-        HgiVulkanConversions::GetImageAspectFlag(texDesc.usage);
     imageSub.baseArrayLayer = isTexArray ? copyOp.sourceTexelOffset[2] : 0;
     imageSub.layerCount = 1;
     imageSub.mipLevel = copyOp.mipLevel;
 
+    // XXX: Vulkan validation demands that only one flag at a time be used 
+    // during the copy operation. Both depth and stencil flags cannot be 
+    // simultaneously passed as aspects to copy.
+    // So, we assume that the user wants to copy depth when the texture is a 
+    // depthStencil texture. If need arises, this part of the implementation 
+    // needs to be re-written such that an aspect flag is passed to this copy 
+    // operation to resolve the discrepancy.
+    imageSub.aspectMask =
+        HgiVulkanConversions::GetImageAspectFlag(texDesc.usage);
+    if (imageSub.aspectMask & VK_IMAGE_ASPECT_DEPTH_BIT) {
+        imageSub.aspectMask = VK_IMAGE_ASPECT_DEPTH_BIT;
+    }
+
     // See vulkan docs: Copying Data Between Buffers and Images
     VkBufferImageCopy region;
     region.bufferImageHeight = 0; // Buffer is tightly packed, like image