Visual Studio doesn't recognize the use of a class member in a static
constexpr unless prefixed with the class name.

BUG=swiftshader:7

Change-Id: I4ffbaa6fc1a43a7294ecdaf426bd7fc2aab5e469
Reviewed-on: https://chromium-review.googlesource.com/380195Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Tested-by: Nicolas Capens <nicolascapens@google.com>