This cl is a minimal implementation of DescriptorPool and
DescriptorSetLayout.

A DescriptorSet itself is just a straight memory pointer.

A DescriptorPool is a memory pool where DescriptorSets are
allocated. The DescriptorPool object itself is little more
than a memory allocator.
A few minor optimizations were made to speed up the default
case:
- The Descriptor pool starts looking for a large enough
  memory block at the end of the pool, which should be optimal
  if the DescriptorSets are allocated linearly
- When allocating multiple DescriptorSets at once, the pool
  first attempts to allocated all of them contiguously. This
  prevents looking for free memory blocks multiple times and
  once again should be optimal if the DescriptorSets are
  allocated linearly

Note: For now, DescriptorSetLayout::GetDescriptorSize() is a
      dummy function which always returns the same size for
      every Descriptor type. This will be implemented properly
      case by case as we add support for new Descriptor types.

Passes all tests in:
dEQP-VK.api.*descriptor*

Bug b/123244275

Change-Id: I2a2e73396e38dae28b59b77243cd8a366b35c12c
Reviewed-on: https://swiftshader-review.googlesource.com/c/24028Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>