Fix VK_KHR_separate_depth_stencil_layouts (#1068)
Fix VK_KHR_separate_depth_stencil_layouts (#1068)

Even though the test app captured and replayed without issue, the state view
showed that `VkAttachmentDescriptionStencilLayout` was not saved for the
render passes that included it. This is caused by the map access
operation returning a copy instead of a reference, making in-place
modification of `renderPass.AttachmentDescriptions[j].StencilLayout`
impossible. The solution here is to assign it to the map after
processing the pNext chain.
google/agi,google/agi,google/agi,google/agi,google/agi,google/agi,google/agi,google/agi