use a compile time constant for kDataAlignment
use a compile time constant for kDataAlignment

const or constexpr should be sufficient for this use but older
versions of gcc fail to expand DECLARE_ALIGNED correctly. Work
around this by using an enum.

Fixed: webm:1660
Change-Id: Ifa4f7585417760f90f9fb28332152019de9f8169

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx