At least for large formats, we assume 16 byte alignment for render
target writes. Requiring 16 bytes in general seems a good idea for
x86_64.

Bug: b/127721996
Change-Id: Ie4d2d9bf71fb8a3d8b58f099785e75a72ff0e181
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26448Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>