This eliminates the duplication between pitchB() and size(), and
ensures that the latter is the full allocation size.

Bug chromium:835299

Change-Id: Icf555ad497fb3b92fd00e9a3e6ced6810b2d310d
Reviewed-on: https://swiftshader-review.googlesource.com/18789Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>