layers: Parse extension buffer to unwrap handles
layers: Parse extension buffer to unwrap handles

The UpdateDescriptorSetWithTemplate extension API contains a
parameter which is a buffer containing Vulkan objects. To get this
through to the driver using the UniqueObjects layer, these handles
must get unwrapped.

Change-Id: I91f573f696eafc00957cbff502aaa8ce96c4e1ba

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers