Make sure all allocations within the pool are 16 bytes aligned

Bug b/137554711

Change-Id: I7e9531b720ae07390e0089768f92fd95c62d6afb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33250Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>