Added functionality so that the CommandPool can allocate and
free command buffer objects.

Bug b/119827933

Change-Id: I190ba3cd7738f2b5e37b196a0abba6d07cfae173
Reviewed-on: https://swiftshader-review.googlesource.com/c/22748Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>