Adding an explicit X8 channel packs the format into 32 bits. This gives
the correct 4-byte alignment instead of a mysterious 3-byte version.

Bug: angleproject:2729
Change-Id: Id38f74db4ba61c500baeb6526a9d14e1d31d4363
Reviewed-on: https://chromium-review.googlesource.com/1140739Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>