This cl adds proper storage and update of descriptor sets.
The added functionality includes:
- The descriptor pool now allocates the proper larger required
  storage needed to actually store descriptor set data.
- Descriptor sets, when allocated, also get initialized with some
  basic header data (set layout) and also some Descriptor data
  when available (a.k.a: immutable samplers)
- Descriptors are currently bindless, since it is simpler as a first
  implementation, but can easily be modified, which is intended to
  be done in the near future. For now, each descriptor set is either
  a VkDescriptorImageInfo, a VkDescriptorBufferInfo or a VkBufferView
- Descriptors can be updated from either a VkWriteDescriptorSet or a
  VkCopyDescriptorSet structure. The update supports writing to
  multiple descriptor sets in a single operation and supports array
  sizes mismatch properly according to the spec

Bug b/123244275

Change-Id: I1e0430e0014e26a304632a4b2b10ad0f69b06180
Reviewed-on: https://swiftshader-review.googlesource.com/c/24910Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>