- Simplified VkNonDispatchableHandle as much as possible so
  that it's just a wrapper over a uint64_t
- Centralized casting logic in vkObject.hpp
- Explicitly changed "Cast" to "vk::Cast" to differentiate
  from vk::<object>::Cast
- Moved VkDescriptorSet operators from VulkanPlatform.h to
  VkDescriptorPool.cpp

Bug b/129979580

Change-Id: I4fe5fcfe56029c21594088d274bae862999597ad
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32928Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>