Copying between different image aspects

Hello,

When copying images, it would be really useful to allow copying between different image aspects, namely copying from a color image to depth or stencil planes of a depthbuffer. This is something that is supported by DirectX 12, and in Vulkan there is no good substitute currently, especially for stencil because stencil can not be exported on a per pixel level on many GPUs.

The use case is that depth or stencil color texture is created in a compute pipeline with appropriate formats (eg. R32_FLOAT for depth, R8_UINT for stencil). Then the depth and stencil color textures would be copied to the corresponding planes of a D32S8 format depth buffer. Example:

```cpp
VkImageCopy copy = {};
copy.dstSubresource.aspectMask = VK_IMAGE_ASPECT_STENCIL_BIT;
copy.srcSubresource.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT;
// ...
vkCmdCopyImage(
commandbuffer,
src_resource, // source image in VK_FORMAT_R8_UINT
VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL,
dst_resource, // destination image in VK_FORMAT_D32_SFLOAT_S8_UINT
VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL,
1,
©
);
```

Apologies if this is not an appropriate issue.

Best regards,
Janos

Start with the VkImageCopy structure and vkCmdCopyImage entry point described in the issue, then review the Vulkan image-aspect rules relevant to color, depth, and stencil planes. Compare the requested behavior with the stated DirectX 12 support and Vulkan limitations. Done means reaching and documenting a specification decision for cross-aspect copies, including the cited depth and stencil cases.

Written by the indexing model from the issue text.